# The hindlimb of Amphicynodon lektorhynchus from the lower Oligocene of the Quercy Phosphorites (France): Highlight of new climbing adaptations of this early arctoid

Axelle Gardin, Manuel Salesa, Gema Siliceo, Mauricio Antón, Juan Francisco Pastor, Louis de Bonis

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Axelle Gardin, Manuel Salesa, Gema Siliceo, Mauricio Antón, Juan Francisco Pastor, et al.. The hindlimb of Amphicynodon lektorhynchus from the lower Oligocene of the Quercy Phosphorites (France): Highlight of new climbing adaptations of this early arctoid. Journal of Mammalian Evolution, 2022, 29, pp.815-843. 10.1007/s10914-022-09621-9. hal-03767991

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Distributed under a Creative Commons Attribution 4.0 International License
This is the author-accepted manuscript of the article: The hindlimb of *Amphicynodon leptorhynchus* from the lower Oligocene of the Quercy Phosphorites (France): Highlight of new climbing adaptations of this early arctoid. *Journal of Mammalian Evolution* 29, 815-843 (2022).
https://doi.org/10.1007/s10914-022-09621-9

The final published version is available at https://link.springer.com/article/10.1007/s10914-022-09621-9.

Supplementary data are available at Zenodo: https://doi.org/10.5281/zenodo.16915117

**The hindlimb of *Amphicynodon leptorhynchus* from the lower Oligocene of the Quercy Phosphorites (France): Highlight of new climbing adaptations of this early arctoid.**

Axelle Gardin<fnref n="1" />*, Manuel J. Salesa<fnref n="2" />, Gema Siliceo<fnref n="2" />,<fnref n="3" />, Mauricio Antón<fnref n="2" />, Juan Francisco Pastor<fnref n="4" />,
Louis de Bonis<fnref n="1" />

**Abstract**

The Itardies locality (MP23, 30–31 Ma), in the Quercy Phosphorites (France), has yielded an impressive amount of fossil remains attributed to *Amphicynodon leptorhynchus*, a genet-sized carnivoran. This species belongs to the family Amphicynodontidae, which was very diversified during the lower Oligocene in Eurasia and whose paleoecology is still poorly known. This study complements our previous analysis of the forelimb of *Am. leptorhynchus*, by describing and discussing several aspects of the functional anatomy of its hindlimb to infer its locomotor abilities and lifestyle. The locomotor adaptations of the hindlimb of *Am. leptorhynchus* portray a talented and agile climber; the great range of mobility of the hip articulation and the shape of the bony structures where the powerful hip abductors, adductors and hamstring muscles attach indicate its ability to firmly grip the substrate, and the knee articulation morphology and the osteological structures supporting the hindlimb extensors suggest powerful extension movements, linked to jumping and fast accelerations, as in half-bounding carnivorans. Its

<footnote n="1">¹ Laboratoire de Paléontologie, Evolution, Paléoécosystèmes et Primatologie (PALEVOPRIM)
UMR 7262 CNRS, Université de Poitiers, Poitiers, France</footnote>

<footnote n="2">² Departamento de Palaeobiología, Museo Nacional de Ciencias Naturales-CSIC, Madrid, Spain</footnote>

<footnote n="3">³ Faculty of Natural Science, Department of Geology and Paleontology, Comenius University,
Bratislava, Slovak Republic</footnote>

<footnote n="4">⁴ Departamento de Anatomía, Facultad de Medicina, Universidad de Valladolid, Valladolid, Spain</footnote>

<footnote>*Corresponding author: Axelle Gardin, axelle.gardin@univ-poitiers.fr</footnote>
hindfoot morphology characterises a mobile articulation, able to perform a hindfoot reversal movement, perhaps even to 180°, as in some extant climbing carnivorans with similar osteological features. This adaptation, highlighted for the first time in a fossil carnivoran, permitted *Am. leptonhynchus* to skillfully descend vertical supports headfirst and to hang by its hindlimbs to reach food and new supports and to capture preys with its forepaws. Although a slow climber model has been suggested based on the morphology of its forelimb, its hindlimb seems to portray a faster and more agile animal, able to climb with control headfirst upwards and downwards and also to perform jumps and bounds between branches or to capture prey, such as the extant ringtail or African palm civet.

**Keywords: Amphicynodon · Locomotion · Palaeoecology · Quercy Phosphorites · Hindfoot reversal · Arctoidea**

**Introduction**

Located southwest of France, the Quercy Phosphorites consists of cavities formed by karstification of Jurassic limestones, which were rapidly filled by continental sediments and remains of the local fauna between the middle Eocene and lower Miocene (Vianey-Liaud and Legendre 1986; Sigé et al. 1991; Laudet and Fernández-Jalvo 1997; Astruc et al. 2000). Within each filling, the faunal association is homogeneous, indicating that the cavity was filled in a few centuries or millennia, and some of these sites have notably served to establish the European mammalian biochronological scale of the continental Paleogene (Aguilar et al. 1997). Each locality offers a snapshot of the Quercynian ecosystem, forming many small windows that, placed end to end, provide a unique insight into the evolution of occidental European fauna and flora over 30 million years. The Quercy Phosphorites represent an ideal framework for the study of the first radiation of the Arctoidea (the clade that includes the extant ursids, pinnipeds, ailurids, mephitids, procyonids, and mustelids), which mainly occurred during the early Oligocene in Europe, through the diversification of the Amphicynodontidae. Representatives of this family, probably paraphyletic, lived during the early Oligocene in Eurasia and possibly between the late Eocene and the Miocene in North America (Cirot and Bonis 1992; Wang and Qiu 2003). They are usually placed within Arctoidea and possibly at the base of Ursoidea (clade formed by ursids and pinnipeds) or Ursida (Cirot 1992; Cirot and Bonis 1992; Wang et al. 2005; Finarelli 2008; Tomiya and Tseng 2016). Their diversification is reflected through the genus *Amphicynodon*, which groups nine mainly European species in lower Oligocene sites (Bonis 1974; Cirot 1992, Cirot and Bonis 1992; Bonis et al. 2019), following their possible immigration from Asia, during the faunal turnover called the “Grande Coupure” at the Eocene-Oligocene transition (Wang et al. 2005). The family Amphicynodontidae has mainly been the subject of taxonomic and phylogenetic studies based on craniodental remains, but their ecology is still poorly known (Cirot 1992; Cirot and Bonis 1992; Wang et al. 2005; Thiery 2012; Gardin et al. 2021).
Among those Quercynian localities, Itardies is of particular interest in the study of Amphicynodontidae, as it has yielded hundreds of cranial and postcranial fossil remains sometimes found in anatomical connection, allowing for the first time the attribution of postcranial material to the species *Amphicynodon leptorhynchus* (Gardin et al. 2021). In extant carnivorans, the ecological requirements are remarkably diverse, leading to various locomotor behaviours: terrestrial generalist, cursorial, climber, digger, and swimmer, thus giving rise to various adaptations reflected in their bone and muscle morphology (Fabre et al. 2013, 2015; Taverne et al. 2018). It is not uncommon for a species to carry out different locomotor activities and use different substrates in its daily life in varying proportions (e.g., Wilson and Mittermeier 2009); therefore, one has to keep in mind that these locomotor categories constitute an artificial and convenient classification useful for ecomorphological studies but do not fully reflect the entire continuous spectrum of behaviours seen in carnivorans. The abundance and remarkable preservation of the fossil remains of *Amphicynodon leptorhynchus* from the Itardies site represent an excellent opportunity to learn, for the first time, about the lifestyle and locomotor behaviour of an Amphicynodontidae. The detailed description and analysis of the forelimb of *Am. leptorhynchus* was already carried out (Gardin et al. 2021). As a summary of the main conclusions yielded by our analysis of the forelimb, *Am. leptorhynchus* was probably a skilful climber weighing around 2 kg. Its anatomical features suggest an ability to increase frictional forces between the hand and the substrate, traction abilities and control of the movement during climbing, as well as grasping abilities. Here we focus on the description and analysis of its hindlimb, compared to other carnivorans exhibiting different locomotor adaptations. In carnivorans, contrary to the forelimb, which is involved in many other activities (foraging, burrowing, grasping, grooming, mating, capturing prey), the hindlimb is more specialised in locomotor activities, and especially in providing impulse during acceleration or running (Davis 1964; Taylor 1970, 1974, 1976; Feeney 1999). Thus, their locomotor behaviour or substrate preference is reflected by their hindlimb morphology through locomotor adaptations. We aim to go further in inferring the locomotor behaviour of *Am. leptorhynchus* and its abilities and its palaeoecology.

# Material and Methods

## 1. Fossil Material

The fossil material of *Am. leptorhynchus* studied here comes from the lower Oligocene site of Itardies, the type locality for the MP23 level of the European Mammalian Biochronological Scale for the Paleogene, which corresponds to 31 – 30 Ma (Aguilar et al. 1997). Discovered and partially emptied by quarrymen during the 19th century, a small quantity of sediment was preserved in this paleokarst (2m² over 50 to 70 cm high), which yielded abundant vertebrate fossil remains. Among the large diversity of taxa, *Am. leptorhynchus* was from far, the most abundant in the cavity: more than a hundred cranial and hundreds of postcranial elements, some of them in anatomical connection, allowing the identification to this species. Other species are mainly represented by marsupials, bats, and rodents, while large ungulates and other
carnivorans are scarce (e.g., Vianey-Liaud 1979, 2003; Rémy et al. 1987; Aguilar et al. 1997; Rémy 2004; Blondel 2005, Sigé and Crochet 2006). Itardies's paleokarst was probably used as a den by *Am. leptorhynchus*, where its remains were found, adults and juveniles, associated with those of its prey, as in extant racoons and ringtails (Endres and Smith 1993; Harrison 2012). The fossils of *Am. leptorhynchus* from Itardies are stored at the paleontological collections of the University of Poitiers, and the list of studied specimens is provided in Online Resource 1. The descriptions and functional analysis are here based on the study of four partial coxal bones, 14 femora, 14 tibiae, two fibulae, 13 tali (also known as astragali), 12 calcanei, 24 naviculars, 27 cuboids, 26 ectocuneiforms, six mesocuneiforms, 12 entocuneiforms, 21 first, 22 second, 25 third, 11 fourth, eight fifth metatarsals, and 19 phalanges, belonging to adults only.

## 2. Extant Specimens Used for Comparisons

The hindlimb of *Am. leptorhynchus* is compared to that of 15 extant carnivorans whose skeletons belong to the collections of the Museum national d'Histoire naturelle (MNHN-Paris), the Museo Anatómico of the Universidad de Valladolid (MAV, Spain), and the Comparative Anatomy Collections of the Museo Nacional de Ciencias Naturales- CSIC (MNCN-CSIC, Madrid, Spain) (Table 1). Those species are classified by substrate preference and locomotor mode categories, described by Van Valkenburg (1987) and Tarquini et al. (2017) (Table 2). For some extant specimens, not all the bones were available; therefore, some figures, measurements, and indices do not record every bone for the compared carnivorans. Moreover, the available specimen of *Procyon lotor* was a juvenile, and the epiphyses were separated from the diaphysis; thus, measurements were not taken on this specimen.

On the other hand, to study the mobility of the ankle in *Am. leptorhynchus*, especially its ability to reverse its feet, measurements were taken based on those selected by Marsh (2019) and Marsh et al. (2021). They studied the ankle morphology of 11 carnivoran species and their respective hindfoot reversal categories with measurements on the distal end of the tibia, talus, and calcaneus. Thus, to predict the hindfoot reversal category of *Am. leptorhynchus* in this study, these same measurements were taken to compare its ankle morphology to that of non-, partial- and full-reversers. Measurements were transformed into ratios to bone length to overcome the varying sizes of the studied taxa for analyses (Online Resource 2).

## 3. Methods

### Descriptions and Morphofunctional Inferences

This paper provides a detailed description of each bone of the hindlimb of *Am. leptorhynchus* and their comparison to those of 15 extant carnivorans with different and known locomotor behaviours, to unravel its locomotor adaptations and paleoecology.

Descriptions are based on the nomenclature of Barone (2010) and the Nomina Anatomica Veterinaria (Schaller and Constantinescu 2007), except for some terms of the ankle bones that follow Chester et al. (2015). As for the forelimb (Gardin et al. 2021), the reconstruction of the
locomotor adaptations and the inference of the locomotor behaviour of this small fossil
arctoid are based on the analysis of: (1) the external bony surfaces, in particular, the
morphology of the articular surfaces to evaluate the mobility of the hindlimb and the relief of
non-articular surfaces (crests, fossae, tuberosities etc.) to infer muscular attachment areas; and
(2) the proportions of the different bones of the hindlimb, through a selection of 18 morpho-
functional indices (Table 4) to understand in- and out-levers in the musculoskeletal system,
relative development of muscular groups and the relative position of bones during stance
phase. Measurements were taken using a digital calliper (Online Resources 1 and 2).
Statistical analyses and graphs (ggplot2 package: Wickham 2016) were performed in RStudio
Version 1.2.5019 (R Core Team 2022) and are based on two datasets.

Hindlimb morpho-functional indices

Eighteen morpho-functional indices were calculated from the hindlimbs of the 14 extant
compared carnivorans (one specimen per species), and a mean value was obtained from the
different bones of *Am. leptorhynchus*. Due to the preparation of the studied skeletons (tendon
and ligaments residues, missing bones), some indices have not been calculated on the skeletons
of *B. astutus* (CRW, PFRL), *N. binotata* (MT3R), and *Gu. gulo* (MTP); in total, 1.6% of the
values are missing in the matrix. These missing values prevent some statistical analyses from
being performed. Therefore, we carried out an imputation of the four missing values thanks to
the missMDA (Josse and Husson 2016) and FactoMineR packages (Lê et al. 2008), allowing
the prediction of several plausible values for each missing data. A graphical check allowed the
validation of the imputed data quality and therefore their use for subsequent statistical analyses.
A principal component analysis (PCA) was performed on the morpho-functional indices to
evaluate which of the extant compared carnivorans have similar hindlimb proportions to those
of *Am. leptorhynchus* (using FactoMineR package, see Lê et al. 2008). The number of studied
principal component (PC) axes is selected using the broken stick model (Borcard et al. 2011).
Phylogenetic relationships among the compared species are projected on the morphospace
thanks to the ggphylomorpho function (Barr 2017). Indeed, part of the morphology of these
carnivorans, including functional features, is likely to be phylogenetically structured.
Therefore, it is important to keep in mind that the values observed in these animals for morpho-
functional indices may be controlled by function and possibly by phylogeny as well. The
phylogenetic tree of the compared carnivorans used projected on the morphospace is based on
the timecalibrated molecular phylogeny presented by Nyakatura and Bininda-Emonds (2012),
except for the Procyonidae, for which we used that of Koepfli et al. (2007) to avoid polytomies.
Our comparative sample does not allow performing an effective discriminant analysis to predict
with accuracy the locomotor category of *Am. leptorhynchus*.

Hindfoot reversal categories discrimination and attribution of *Am. leptorhynchus*

One PCA was performed on all the ratios from the tibia, talus and calcaneus, to evaluate which
of the extant carnivorans have similar ankle proportions to those of *Am. leptorhynchus* (using
FactoMineR package, see Lê et al. 2008). The number of studied PC axes is selected using the broken stick model (Borcard et al. 2011). Then, four leave-one-out cross-validated linear discriminant analyses (LDA) were conducted in order to predict the hindfoot reversal ability of *Am. leptorhynchus*. For the first LDA based on the whole ankle articulation, we used the two first PC axes as predictors, to decrease the number of variables. For the three others LDAs, for each bone individually, we used the five to six corresponding ratios. The LDAs were computed using the 'lda' function ('MASS' package, see Ripley et al. 2013). The leave-one-out cross-validation removes one specimen from the sample and predicts its category according to a model built with the remaining specimens. In this study, the model is built based on observations on extant carnivorous made by Marsh et al. (2021). In their analysis, they placed Ailurus fulgens as a non-reverser. However, considering a mention in the literature of its ability to fully reverse its feet (Makungu et al. 2014) and the observations of at least partial reversal on living specimens (Gardin pers. obs.), *Ai. fulgens* is considered within the full-reversers category for this study. Finally, the hindfoot reversal category of *Am. leptorhynchus* is predicted from those models and is based on 13 tibiae, 10 tali and eight calcanei. The prediction with the model including all the data from the three bones is based on the mean values for each ratio calculated for *Am. leptorhynchus*. Analysing all the specimens makes it possible to take into account the significant variability of the studied characters within each species, but it is noteworthy that procedure can lead to an exaggeration of the predictions.

# Results

## 1. Description and comparison with extant carnivorous

### Coxal Bone

The rectangular, dorsoventrally narrow, and flat ilium of *Am. leptorhynchus* is similar in shape to that of *B. astutus*, *Ga. vittata* (although it flares more laterally), *Ge. genetta*, *Ma. foina*, *N. binotata* and *Po. flavus*, as illustrated by their low values in IRD comprised between 0.34 and 0.43 (Fig. 1a; Table 5). In the other compared carnivorous, the ilium is more concave, dorsoventrally wider, and flares laterally, especially the ventral border of the ilium, thus having greater values in IRD between 0.47 (Fig. 1b-e; Table 5). Like *Ge. genetta*, the lateral face of the ilium of *Am. leptorhynchus* shows dorsal concave surface for the origin of the m. gluteus medius (Fig. 1f) (Taylor 1976). It is delimited ventrally by a wide and convex gluteal line, and the m. iliacus originates on the ventral margin of the ilium as described in extant carnivorous (Fig. 1f) (Taylor 1976; Barone 2010; Ercoli et al. 2013). In the other climbing carnivorous and *Ga. vittata*, the ventral border of the ilium is thicker and flatter. The craniomedial face of the ilium of *Am. leptorhynchus* shows a shallow concave surface that may be the insertion area for the m. iliocostalis lumborum, as recorded in *C. lupus familiaris* and *Ga. cuja* (Barone 2010; Ercoli et al. 2013). This muscle attachment surface is relatively reduced in *Ailuropoda melanoleuca*, while it is not mentioned in the studies on the myology of *Ai. fulgens*, *B. astutus* and *Po. flavus* (Davis 1964; Fisher et al. 2008; Liu et al. 2016; Marsh et al. 2021). In *Am*.
*leptorhynchus*, the auricular surface on the medial face of the ilium is crescentic, with two cranially oriented dorsal and ventral lobes, with an angular caudal border. This configuration is also seen in *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *Ge. genetta*, *Gu. gulo*, *Ma. foina*, *N. binotata* and *Pr. lotor*, while it is smaller and more circular in the compared canids. The dorsal part of the auricular surface is carried by the dorsocaudal iliac spine, which is very marked and strongly protruding medially, as in *Ai. fulgens*, *B. astutus*, *Na. nasua*, *Po. flavus*, *Pr. lotor* and *T. ornatus*. The craniodorsal border of the ilium of *Am. leptorhynchus*, corresponding to the craniodorsal iliac spine, is thick and rounded, providing an origin area for the m. gluteus superficialis, like that of *Ai. fulgens*, *Ga. vittata*, *Ge. genetta*, *Ma. foina*, *Na. nasua*, *N. binotata* and *Po. flavus* (Fig. 1a, b, f) (Fisher et al. 2008; Ercoli et al. 2013; Liu et al. 2016). It is more dorsally developed in the compared canids (where attach m. sartorius and m. obliquus externus, see Barone 2010), *Gu. gulo* and *T. ornatus* (Fig. 1c). The body of the ilium is short and thick in *Am. leptorhynchus*, as seen in most of the climbing carnivorans, while it is longer, narrower and constricted in *Ai. fulgens* and the other terrestrial carnivorans (Fig. 1).

Cranial to the acetabulum, there is a well-developed tubercle, the ventrocaudal iliac spine, with two circular scars, providing origin areas for the m. rectus femoris, similar to that observed in *Ai. fulgens*, *B. astutus*, *Ge. genetta*, *N. binotata* and *Po. flavus*, which are however less developed than in *Am. leptorhynchus* (Fig. 1) (Taylor 1976; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013; Liu et al. 2016). The acetabulum of *Am. leptorhynchus* is a widely open and circular cavity. The articular surface is oval and shallow, being very similar to that of climbing carnivorans, compared to terrestrial ones and *T. ornatus*, which have a deeper acetabulum with a buttressed dorsal border (Fig. 1). Caudal to the acetabulum, the ischial spine strongly dorsally protrudes, as in *Ge. genetta*, *Ma. foina*, *N. binotata* and *Po. flavus* (Fig. 1) and it is relatively caudally located, as in *Ge. genetta* and *N. binotata*. It is less developed in the other climbing carnivorans, and even less in the terrestrial carnivorans. Caudally to this spine, there is an abrupt passage for the tendon of m. obturatorius internus. The m. gemellus caudalis usually originates slightly cranial or caudal to the ischial spine in *Ai. fulgens*, *B. astutus*, viverrids, and *C. lupus familiaris*; while the m. gemellus cranialis originates cranial to the ischial spine (Taylor 1976; Fisher et al. 2008; Barone 2010; Liu et al. 2016).

The part formed by the ischium and the pubis represents the half of the coxal bone of *Am. leptorhynchus* (Fig. 1a). Although it is not fully preserved in the fossil specimens, it seems to have a rectangular outline similar to that of *B. astutus* and *Ga. vittata*, while it is generally more quadratic to triangular in the compared climbing carnivorans. This shape is due to the well-developed ischium, that extends more ventrally (Fig. 1a). The obturator foramen is wide and has an oval aperture, caudally dorsoventrally larger. The caudal extremity of the ischium is buttressed and wide craniocaudally, thus providing a large and flat origin area for the m. semimembranosus, as in *B. astutus* (Fig. 1a, e) (Taylor 1976; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013; Liu et al. 2016; Marsh et al. 2021). The dorsocaudal extremity of the ischium ends by the ischial tubercle where the m. biceps femoris, the m. semitendinosus, and the m.
quadratus femoris originate in extant carnivorans; it protrudes dorsally, as in *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *Ge. genetta*, *Ma. foina*, and *N. binotata* but more than in *Po. flavus* and less than that of the compared canids, *Me. meles*, *Na. nasua*, and *T. ornatus* (Fig. 1) (Taylor 1976; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013; Liu et al. 2016; Marsh et al. 2021). The pelvic symphysis is not fully preserved in any specimen of *Am. leptorhynchus* but seems to be relatively long according to the proportions of the preserved coxal bone. The pubis of *Am. leptorhynchus* is a thick bone, as seen in climbing species and *Ga. vittata*, providing an origin area for the adductor musculature (e.g., m. adductor femoris and m. pectineus) (Fig. 1a, f) (Davis 1964; Taylor 1976; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013; Liu et al. 2016; Marsh et al. 2021).

## Femur

The femur of *Am. leptorhynchus* is similar in robustness to that of *B. astutus*, *Po. flavus*, and *T. ornatus*, with a straight diaphysis and a circular section (Fig. 2a). As in *Ai. fulgens*, *B. astutus*, and *Gu. gulo*, the caudal face of the diaphysis shows two smooth lines emerging from the third trochanter. One line is going medially, while the other one goes straight distally, thus forming a large caudal surface (facies aspera) where the adductor muscles attach (Fig. 2). The distocaudal face of the diaphysis is flat in *Am. leptorhynchus*, as in *B. astutus* and *Po. flavus*. Proximal to the lateral epicondyle, there is a small tuberosity, more or less developed depending on the specimens, similar to that of the compared canids, *Ge. genetta*, *Na. nasua*, and *Po. flavus*, probably linked to the origin area of the lateral head of the m. gastrocnemius (Davis 1964; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013; Liu et al. 2016; Marsh et al. 2021).

In *Am. leptorhynchus*, the femoral head is spherical, with the articular surface extending proximolaterally onto the neck, as in *Po. flavus*; this extension is more marked than that of the compared climbing carnivorans, and even more than that of the terrestrial species (Fig. 2). The femoral head shows a FHI equal to 1 in *Am. leptorhynchus*, close to the values observed in *B. astutus*, *Ai. fulgens*, *Ga. vittata*, *N. binotata*, *Na. nasua*, and *Po. flavus*, while it is less spherical in the other studied taxa (Table 5). As in *B. astutus*, *N. binotata*, and *Po. flavus*, the femoral neck of *Am. leptorhynchus* is robust and short, as an intermediate between the slender ones of *Ai. fulgens*, *Ge. genetta*, and *Ma. foina*, and the more robust of *Na. nasua* and *Pr. lotor*. It is relatively longer in *Ga. vittata* and *Me. meles* and slenderer in the compared canids (Fig. 2). In *Ga. vittata* and *Me. meles*, the proximal epiphysis of the femur differs from that of the climbing taxa by showing a different configuration: the femoral head is relatively much larger and protrudes more proximally than the greater trochanter. As in *Ai. fulgens*, *B. astutus*, *C. lupus*, *Ma. foina*, *N. binotata*, and *Po. flavus*, lateral to the femoral head, the greater trochanter of Am. is located at the same level as the femoral head proximally, providing a rough lateral surface for the attachment of the m. gluteus profundus, m. gluteus medius, and m. gluteus superficialis, as described in extant carnivorans (Fig. 2) (Davis 1964; Taylor 1976; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013; Liu et al. 2016; Marsh et al. 2021). *Am. leptorhynchus* shows similar values of GLU to those of *B. astutus*, *Ga. vittata*, *Ge. genetta*, *N. binotata*, and *Po. flavus*, very
low (between 0.09 and 0.12) compared to those of the other compared taxa (between 0.15 and 0.24) (Table 5). Distally, close to the femoral head, the lesser trochanter of the femur of *Am. leptonhynchus* is similar to that of *B. astutus*, *Ge. genetta*, *Gu. gulo*, and *N. binotata*, being more developed and more medially protruding than that of the other compared carnivorans, but less than in *Ai. fulgens* (Fig. 2). This trochanter represents the insertion area of the m. iliopsoas in extant carnivorans (Davis 1964; Taylor 1976; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013; Liu et al. 2016). Contrary to most of the taxa, the lesser trochanter of the femur of *Am. leptonhynchus* is not connected to the greater trochanter by a well-marked intertrochanteric crest but has an arrangement similar to that of *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *Ge. genetta*, *Ma. foina*, *N. binotata*, and *Po. flavus* with a smooth crest, which provides a large and flat origin surface for the m. quadratus femoris, as described for extant carnivorans (Fig. 2) (Davis 1964; Taylor 1976; Fisher et al. 2008; Barone 2010; Liu et al. 2016; Marsh et al. 2021).

Cranially, the trochlea of the distal epiphysis in *Am. leptonhynchus* is relatively high, with a more or less wide, depending on the specimens, and shallow patellar articular surface, being similar to that of *B. astutus* and *N. binotata*; it is intermediate between the high trochlea of the terrestrial species, and the low one of the other climbing species and *Ga. vittata* (Fig. 2). However, its overall morphology is more similar to that of climbing taxa and *Ga. vittata* with low values in FTD (between 0.08 and 0.24), being wide and shallow with two blunt lateral ridges, while cursorial and semi-fossorial taxa exhibit greater values in FTD (between 0.26 and 0.46) (Fig. 2; Table 5). The two condyles are equivalent in size and project caudally from the femoral diaphysis, as in the climbing taxa, *Ga. vittata* and *Me. meles* (low value in FCP between 0.14 and 0.18), while they are more caudally projected and of different size in the compared canids (with a greater FCP between 0.19 and 0.21) (Fig. 2; Table 5). As seen in *Ai. fulgens*, *Ge. genetta*, *Ma. foina*, and *N. binotata*, the lateral face of the lateral condyle is characterized by a deep and elongated fossa for the origin of the m. popliteus, and a smaller one located more cranially for the m. extensor digitorum longus as described for extant carnivorans (Fig. 2f) (Davis 1964; Taylor 1976; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013). The medial face of the medial epicondyle is marked by a large depression that may have served for the origin of the medial head of the collateral ligament (Fig. 2f) (Davis 1964; Fisher et al. 2008; Liu et al. 2016; Marsh et al. 2021).

## Tibia

In *Am. leptonhynchus*, the crural index (CI = 0.94) is lower than in *Ge. genetta*, *Ny. procyonoides*, and *V. vulpes* (1.06–1.07), and greater than in *Me. meles* and *T. ornatus* (0.79), being similar to that of the other compared carnivorans (Table 5). The tibia of *Am. leptonhynchus* is a slightly laterally curved bone as in the compared canids, *B. astutus*, *Ga. vittata*, *N. binotata*, and *Po. flavus* (Fig. 3). It is similar in robustness to that of *Ge. genetta* and *Po. flavus*, while it is slenderer in *Ai. fulgens*, *B. astutus*, *Ma. foina*, and *N. binotata*, and more robust in the other compared carnivorans (Fig. 3). The tibia of *Am. leptonhynchus* is circular in section at midshaft, while it is triangular in the compared canids.
In *Am. leptorhynchus*, the tibial proximal epiphysis is relatively mediolaterally wider than craniocaudally in climbing carnivorans and *Galictis vittata*, with low values in THI between 0.80 and 0.86 (Table 5). These values are greater in the other terrestrial carnivorans (THI between 0.89 and 1.10). In *Am. leptorhynchus*, the lateral condyle is slightly larger than the medial one and is convex, as in *N. binotata* and *Po. flavus*. It shows a large articular facet for the fibula on its laterocaudal border, as in *B. astutus*. *N. binotata*, and *Po. flavus* (Fig. 3). On the other hand, the medial condyle is concave. The two condyles are separated by a smooth intercondylar eminence, as in climbing carnivorans and *Ga. vittata*, a structure that is proximally higher in the other terrestrial taxa (Fig. 3). On the cranial face of the proximal epiphysis, the tibial tuberosity is smooth and does not strongly protrude cranially as seen is the compared canids, *Ga. vittata* and *Gu. gulo*. This surface provides an attachment area for the patellar ligament, which corresponds to the distal part of the tendon of m. quadriceps femoris (Taylor 1976). It continues distally in *Am. leptorhynchus*, along the proximal third of the diaphysis, forming the tibial crest, with a distal extension similar to that of the climbing carnivorans and *Ga. vittata* (TTI between 0.31 and 0.38), while it ends more distally in *Me. meles* (TTI equal to 0.49) and more proximally and extends more cranially in the compared canids (TTI between 0.22 and 0.29) (Fig. 3; Table 5). As in *Ai. fulgens*, *B. astutus*, *N. binotata* and *Po. flavus*, there are two scars on the distal part of the tibial crest of *Am. leptorhynchus*, where the m. gracilis and the m. semitendinosus attach, and an area on the proximomedial face of the tibial tuberosity for the m. sartorius as described in studies on the myology of some carnivorans (Fig. 3f) (Davis 1964; Taylor 1976; Ercoli et al. 2013; Liu et al. 2016). These scars are not visible in the other taxa or are represented by a rough area. Mediocaudal to the medial condyle of the proximal epiphysis of the tibia of *Am. leptorhynchus*, there is a depression, as in *B. astutus* and *N. binotata*, on which inserts the m. semimembranosus in extant carnivorans (Davis 1964; Taylor 1976; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013; Liu et al. 2016). As in *B. astutus*, *N. binotata* and *Na. nasua*, distal to this surface, there is a well-marked and proximodistally elongated scar for the insertion of the medial collateral ligament (Fisher et al. 2008; Liu et al. 2016). It delimits the insertion area for the m. popliteus cranially, which is relatively large, as in *Ai. fulgens*, *Ma. foina*, *Na. nasua* and *T. ornatus* (Fig. 3a, f) (Davis 1964; Taylor 1976; Fisher et al. 2008; Barone 2010; Liu et al. 2016; Marsh et al. 2021). The lateral face of the proximal epiphysis shows a deep origin area for the m. tibialis cranialis as described in some extant carnivorans (Fig. 3f) (Taylor 1976; Fisher et al. 2008; Liu et al. 2016; Marsh et al. 2021). In caudal view, two crests emerge from the condyles, thus delimiting the origin area for the m. tibialis caudalis, the m. flexor digitorum lateralis and m. flexor digitorum medialis, as reported for other carnivorans (Fig. 3f) (Taylor 1976; Fisher et al. 2008; Barone 2010; Ercoli et al. 2013; Liu et al. 2016).

The distal epiphysis is relatively wide as in *Ai. fulgens*, *B. astutus*, *N. binotata*, and *Po. flavus*, but not so much as in *Ga. vittata* and *Me. meles*. The distal articulation surface shows two shallow fossae separated by a smooth diagonal ridge. As in climbing carnivorans and *Ga. vittata*, the relief in this articular surface is low, while in the other terrestrial carnivorans the
relief is well marked, with a strong ridge and deep lateral fossa. In *Am. leptorhynchus*, the lateral distal articular surface for the talus goes proximally as in *B. astutus*, *N. binotata* and *Po. flavus*. The medial face of this epiphysis presents a wide passage for the tendon of the m. tibialis caudalis, delimited by two caudodistally oriented ridges as described in extant carnivorans (Fig. 3a) (Fisher et al. 2008; Ercoli et al. 2013; Liu et al. 2016). This tendon runs along the medial side from the caudal to the cranial faces. The passage for this tendon is smoother in the compared canids, *Me. meles*, *Pr. lotor*, and *T. ornatus*. This face of the epiphysis is characterized by the medial malleolus, robust and developed distally, as in *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *Gu. gulo*, *Ma. foina*, *Na. nasua*, *N. binotata*, *Po. flavus*, and *Pr. lotor*, while it is shorter is the other compared carnivorans (Fig. 3). The lateral face of the distal epiphysis of the tibia shows a large crescentic fibular notch, craniocaudally elongated, located just proximally to that for the talus, as in *Ai. fulgens*, *Ga. vittata*, *Ge. genetta*, *Ma. foina*, *Na. nasua*, and *Po. flavus* while this facet is reduced in the compared canids.

## Fibula

The proportions of the fibula of *Am. leptorhynchus* are like that of *B. astutus*, *Ga. vittata*, *Gu. gulo*, *Me. meles*, *Na. nasua*, and *Pr. lotor*, relatively more robust than that of the other compared carnivorans (except that of *T. ornatus*, which is more robust) (Fig. 4).

The fibular head is large, triangular, and craniocaudally elongated (Fig. 4a). On the medioproximal face, there is a large elliptical articular facet for the tibia, similar to that of the climbing carnivorans and *Ga. vittata*, while it is smaller in the compared canids (Fig. 4). This facet is surrounded by three well-developed tubercles, like those of *B. astutus*, *Ge. genetta*, *Ma. foina*, *N. binotata*, and *Me. meles* (Fig. 4a). The cranial tubercle serves for the insertion of the lateral collateral ligament and the m. fibularis longus; the medial tubercle is the attachment area for the m. flexor digitorum medialis; and finally, the caudal tubercle provides an origin area for the m. soleus in extant carnivorans (Fig. 4f) (Taylor 1976; Fisher et al. 2008; Ercoli et al. 2013; Liu et al. 2016; Marsh et al. 2021).

The diaphysis of the fibula underwent a flattening in the two available fossil specimens of *Am. leptorhynchus*. As in *Ge. genetta*, two strong crests are emerging from the caudal proximal tubercle, joining at the proximal third of the diaphysis. These crests form a delimited surface for the origin of the m. tibialis caudalis. On the distal third of the diaphysis, there are two caudal crests, one going laterally and the other medially, forming a flat surface, as in *Me. meles*, *Na. nasua*, *N. binotata*, *Pr. lotor*, and *T. ornatus*.

The distal epiphysis is mediolaterally large, reaching the same distal level as that of the medial malleolus of the tibia (Fig. 4a). The medial face of the distal epiphysis shows two articular facets, similar to that of *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *Ge. genetta*, *Ma. foina*, *Na. nasua*, and *Po. flavus*. The proximal one is small and triangular, mediocaudally and proximally oriented; it articulates with the tibia. The second facet is for the articulation with the talus; it is very large, elliptical, mediocaudally and distally oriented, and it occupies most of the medial
face of the distal epiphysis. The caudal extremity of the articulation surface with the talus ends by a small lateral tubercle. Cranial to this tubercle, there is a second well-developed tubercle, separated from a proximolateral third tubercle by an oblique and deep groove for the tendon of the m. fibularis brevis and m. fibularis longus, as seen in *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *Ge. genetta*, *Ma. foina*, *N. binotata*, *Me. meles*, *Na. nasua*, and *Pr. lotor* (Fig. 4a, b,e) (Fisher et al. 2008). This groove is horizontal in the compared canids and *Gu. gulo*.

## Tarsals

The talus of *Am. leptorhynchus* is similar in proportions to that of *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *N. binotata*, *Na. nasua*, and *Pr. lotor*; it is more robust than that of *Ge. genetta*, *Ma. foina*, and *Po. flavus* but less than that of the other compared extant carnivorans. The talar trochlea is very convex and hemispherical, as in the compared canids and *Ge. genetta*, more than in *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *Ma. foina*, *Me. meles*, *N. binotata*, *Na. nasua*, and *T. ornatus*, but less than that of *Gu. gulo* and *Po. flavus* (exhibiting the highest TACV values in our sample) (Table 5). As seen in *Ai. fulgens*, *B. astutus*, *Ge. genetta*, *Ma. foina*, and *Po. flavus*, the medial lip is rounded, wide while the lateral lip is angular and protrudes more proximally. On the opposite, the medial lip is more angular in the compared canids, *Ga. vittata*, *Me. meles*, and *T. ornatus*. *Am. leptorhynchus* exhibits relatively average values in TACS (mean 0.49), being similar to those observed in the compared canids, *Ga. vittata*, *N. binotata*, *Na. nasua*, and *T. ornatus* (from 0.4 to 0.53) indicating a relatively symmetrical trochlea, while this index is greater in *B. astutus* and *Me. meles* (from 0.59 to 0.65) and lower in *Ai. fulgens*, *Gu. gulo*, *Ma. foina*, and *Po. flavus* (0.3 to 0.39) (Fig. 5; Table 5). In *Am. leptorhynchus*, the two lips are separated by a shallow relief, as in *B. astutus*, *C. lupus*, *Ga. vittata*, *Ge. genetta*, *Gu. gulo*, *Me. meles*, *N. binotata*, and *Na. nasua* (TARD between 0.18 and 0.24), while it is deeper in *Ny. procyonoides* and *V. vulpes* (TARD between 0.40 and 0.42) and shallower in *Ai. fulgens*, *Ma. foina*, *Po. flavus*, and *T. ornatus* (between 0.09 and 0.14) (Fig. 5; Table 5). Contrary to *Me. meles* and *Po. flavus* (shallow pit), the curvature of the trochlea of *Am. leptorhynchus* is not plantarly interrupted by a talar foramen (Fig. 5a, d). However, the two lips protrude more plantarly in *Am. leptorhynchus*, thus forming a marked groove, extended onto the plantar face of the talus, oblique in relation to the trochlea axis for the passage of the tendon of the m. flexor digitorum lateralis as described in some mammals (Fig. 5) (Fisher et al. 2008; Ercoli et al. 2013; Chester et al. 2015; Liu et al. 2016). In the studied terrestrial taxa, this groove is in the same axis as the trochlea, thus extending part of the trochlea. As in the climbing carnivorans, there is a smooth tubercle on the plantar extremity of the medial lip, less marked than in *Pr. lotor* (Fig. 5). The dorsal part of the trochlea is slightly less curved than the plantar part and its curvature extends smoothly on the talar neck. The plantar face of the talus shows two articular facets for the calcaneus, separated by a shallow and wide sulcus, which is deeper and perforated in the compared canids, *Ga. vittata*, *Gu. gulo*, *Me. meles*, *Pr. lotor*, and *T. ornatus*. In *Am. leptorhynchus*, the laterally located plantar facet, called the ectal facet, is wide, concave and dorsoplantarly elongated (Fig. 5a). Its caudal half is lateroplantarly oriented, while its cranial
half is plantarly oriented and slightly convex. As in the climbing taxa and *Ga. vittata*, this surface is smooth, while it is more angular and rougher the other taxa, with an abrupt separation between the two halves. It is more laterally oriented in *N. binotata* and *Ga. vittata*. In *Am. leptonhynchus*, the medially located plantar facet, the sustentacular facet, is very large, elliptical in shape, flat to slightly convex and medioplantarly oriented. The particularity of this facet in *Am. leptonhynchus* is its extension on the medial face of the talar neck, very close to the talar head, as in *Ai. fulgens*, *B. astutus*, *Ga. vittata*, and *Po. flavus*. The medial face of the talus of *Am. leptonhynchus* possesses a shallow cranial and oblique groove, in which articulates the well-developed medial malleolus of the tibia, very similar to that of *Ai. fulgens*, *Gu. gulo*, *N. binotata*, and the procyonids (Fig. 5a). This depression extends plantarly in the compared canids, *Ga. vittata* and *T. ornatus*. The plantar half of the medial face is medially convex. As in *Ga. vittata*, *Ma. foina*, and *Po. flavus*, the lateral face of the talus of *Am. leptonhynchus* is relatively flat, with a shallow caudal depression, which is more marked in the other taxa. The talar trochlea is separated from the talar head by a relatively thick neck, as an intermediate between that of *B. astutus* shorter and stouter and that of *N. binotata* and *Po. flavus* which is very elongated (Fig. 5a). This neck is constricted at the base and dorsomedially projected, as seen in the climbing carnivorans and *Ga. vittata*, while it dorsally protrudes and is not constricted in *C. lupus*, *T. ornatus*, and *V. vulpes*. The talar head of *Am. leptonhynchus* is large, representing a third of the bone length, like in *B. astutus* and *N. binotata*. It is mediolaterally wide, bulbous and shows an oval outline, as in *Ai. fulgens*, *B. astutus*, *N. binotata*, and *Po. flavus* (Fig. 5). The distal extremity of the talar head extends proximally all around the head, similarly to the climbing taxa and *Me. meles*.

The calcaneus of *Am. leptonhynchus* is similar in robustness to that of most of the compared climbing carnivorans, *Ga. vittata* and *Me. meles*, more gracile than that of *T. ornatus* and more robust than that of the compared canids and *Ge. genetta*. The tuber calcanei of *Am. leptonhynchus* is long relative to the calcaneus length (CTC = 0.68), being similar to *Na. nasua*, *Ma. foina*, *Gu. gulo*, *Me. meles*, *T. ornatus*, and *C. lupus* (0.68-0.74), while it is shorter in *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *Ge. genetta*, *N. binotata*, *Ny. procyonoides*, *Po. flavus*, and *V. vulpes* (0.59-0.65) (Fig. 5; Table 5). It is also relatively robust, like that of *Ge. genetta*, *Na. nasua*, *Ny. procyonoides*, *Po. flavus*, and *T. ornatus*, while the other compared species have a more gracile tuber calcanei (Fig. 5). On the proximal extremity of the tuber calcanei, there is a shallow groove delimited by a medial tubercle and a slightly smaller lateral one, where the tendo calcaneus attaches (m. gastrocnemius, m. flexor digitorum superficialis and m. soleus in *Ai. fulgens*, *B. astutus*, and *Ga. cuja* see Fisher et al. 2008; Ercoli et al. 2013; Liu et al. 2016; and only the m. gastrocnemius in *Po. flavus* see Marsh et al. 2021). As in *Ga. vittata*, *N. binotata*, and *B. astutus*, the fibular tubercle is well-developed in *Am. leptonhynchus* (having relatively high CRW values), placed proximally compared to that of the other carnivorans (Fig. 5). This tubercle shows a shallow plantar depression, which is deeper in the terrestrial taxa and flatter in *T. ornatus*, providing an origin area for the m. quadratus plantae in extant carnivorans (Fisher et al. 2008; Ercoli et al. 2013; Liu et al. 2016). There is also a marked groove on the
lateral extremity of the tubercle, as seen in *B. astutus*, *Ga. vittata*, *Gu. gulo*, *Na. nasua*, *N. binotata*, and *Po. flavus*. The distal surface of the calcaneus articulating with the cuboid is slightly inclined dorsally, slightly concave, with a slight mediolateral elongation and a small medial tubercle, very similar to *Ai. fulgens*, *Ma. foina*, and *Po. flavus*. The sustentaculum tali is very prominent in *Am. leptonhynchus*, placed distomedially to the calcaneal ectal facet, as in *Ai. fulgens*, *Ga. vittata*, *Ge. genetta*, *Ma. foina*, *Me. meles* *N. binotata*, *Po. flavus*, and *Pr. lotor* (Fig. 5). It is at the same level as this latter facet in the compared canids, and in an intermediate position in the other compared taxa (Fig. 5). There is a dorsal rectangular facet articulating with the sustentacular facet of the talus, facing dorsodistally, as in *Ai. fulgens*, *Ge. genetta* *N. binotata*, and *Po. flavus*, while it is dorsally oriented in *Ga. vittata*. The plantar face of the sustentaculum tali is marked by a deep groove, serving for the passage of the tendon of the m. flexor digitorum lateralis in extant carnivorans (Fisher et al. 2008; Ercoli et al. 2013). In *Am. leptonhynchus*, the calcaneal ectal facet lies in a different plane than the facet of the sustentaculum tali, contrary to that of the compared canids (Fig. 5). Located proximally to the latter facet, the calcaneal ectal facet of *Am. leptonhynchus* is elliptical, proximo-distally elongated, as in *N. binotata* and *T. ornatus* (CEFL between 0.35 and 0.38) and extends more proximally than in the other compared taxa (Fig. 5; Table 5). The articulation shows two orientations linked by a convex and continuous curvature: the proximal half has a medial orientation, extending proximally on the tuber calcanei as in *Ai. fulgens*, *N. binotata* and *Po. flavus*, while the distal half has a dorsomedial orientation (Fig. 5). It is more convex and continuous than in *Na. nasua*. The proximal part of the facet is concave in compared canids, *Gu. gulo* and flatter in *Ai. fulgens*, *Ge. genetta* and *Pr. lotor*. The two facets of the dorsal face of the calcaneus are separated by a wide and shallow sulcus in *Am. leptonhynchus*, as well as in the climbing carnivorans and *Ga. vittata*, while it is narrower and deeper in the other compared taxa (Fig. 5).

The proximal rectangular facet of the navicular is smooth and concave, slightly elongated dorsoplantarly, being very similar to that of *Ai. fulgens* and *Po. flavus* (Fig. 6a-d). It is flatter in *Ga. vittata*, *Na. nasua*, *Ny. procyonoides*, *Me. meles*, and *Pr. lotor* and more dorsoplantarly elongated in *Ge. genetta* and *Ma. foina*. As in climbing carnivorans, this surface is delimited by the proximally protruding margins, especially the dorsal one, thus surrounding the talar head. This bone is less high proximodistally than that of the compared canids. On the medioplantar corner, there is a tuberosity protruding proximally, and another rough one on the lateroplantar corner of the bone, protruding distally (Fig. 6a). These tuberosities are not well marked in *Na. nasua* and *Pr. lotor*, while there are in the terrestrial taxa. The distal face of the bone is divided into three continuous and convex articular surfaces for the cuneiforms as in the climbing carnivorans and *Me. meles*, while it is flatter in the compared canids and *Gu. gulo*. The dorsolateral quarter of this face corresponds to the articular facet for the ectocuneiform, which is slightly convex and dorsoplantarly elongated (Fig. 6a-d). The medial half of the distal face the navicular forms the articular facets, not clearly separated, dorsally for the mesocuneiform and plantarly for the entocuneiform, as seen in *Ai. fulgens*, *Ge. genetta*, *Ma. foina*, and *Po*.
*flavus* (Fig. 6a-d). The lateral side of the bone shows a central articular surface for the cuboid, dorsoplantarly elongated, flat and rectangular.

The cuboid of *Am. leptorhynchus* is similar to that of *Ai. fulgens*, *Ge. genetta*, and procyonids, while it is more proximodistally flattened in *Gu. gulo*, *Ma. foina* and *T. ornatus* and more elongated in the terrestrial taxa (Fig. 6e-h). The proximal surface of this bone is inclined, as in the climbing carnivorans and *Ga. vittata*, except *Ge. genetta* and the other terrestrial taxa, in which it is straight (Fig. 6e-h). This facet is slightly convex, mediolaterally elongated and crescentic, showing a small articular facet for the talar head on its medial margin, as in *Ai. fulgens* and *Po. flavus*. This facet is more elongated in *Gu. gulo* and *Pr. lotor*. The distal surface, which articulates with the Mt IV and Mt V, is relatively small, crescentic, concave and dorsoplantarly elongated, very similar to that of *Po. flavus*. The plantar face of the cuboid of *Am. leptorhynchus* shows a deep central depression in its proximal half (Fig. 6e-h). This depression is delimited distally by a well-developed and prominent tuberosity, constituting part of the proximolateral wall of an osseofibrous plantar tunnel, which acts as a passageway for the tendons of the m. tibialis caudalis, the m. flexor digitorum medialis and the m. flexor digitorum lateralis (Fisher et al. 2008). Distal to this tubercle, there is a groove for the passage of the tendon of m. fibularis longus (Fig. 6eh) (Fisher et al. 2008). This configuration is very similar to that of the climbing carnivorans, while the plantar tubercle is less marked in the terrestrial taxa. The medial face of the cuboid shows two articular surfaces (Fig. 6e-h). Placed centrally in this medial face, the larger crescentic articular surface serves for articulation with the proximal face of the ectocuneiform and with the navicular, similar to that of *Ai. fulgens*, *Ga. vittata*, *Po. flavus*, and *Pr. lotor*. The second surface also articulates to the ectocuneiform; it is very small and located in the mediodistal corner. This latter is not visible in all the specimens, probably due to the state of preservation of the bone. It is absent in *Gu. gulo*, *Ny. procyonoides*, and *V. vulpes* but present in the other compared taxa, with a more or less important degree of development.

The overall shape of the entocuneiform of *Am. leptorhynchus*, very similar to that observed in *Ma. foina* and *Po. flavus*, is triangular to rectangular, proximodistally elongated and mediolaterally compressed (Fig. 6i-j). The proximoplantar margin is less projected than that of *Ai. fulgens*, *Na. nasua*, and *Pr. lotor* (to a lesser extent in this latter). The articular surfaces, proximally with the navicular, and distally with the Mt I, are strongly concave and narrow, dorsoplantarly elongated.

The mesocuneiform of *Am. leptorhynchus* is very similar to that of *Ma. foina* and *Ga. vittata*, being a quadratic narrow bone, compressed mediolaterally and not very dorsoplantarly elongated as seen in the other taxa (Fig. 6 m-o). It is far more robust mediolaterally in *Gu. gulo* and *Pr. lotor*, and proximodistally flattened in *T. ornatus*. The proximal and distal articular facets of the mesocuneiform of *Am. leptorhynchus*, articulating respectively with the navicular and the Mt II, have a triangular outline dorsoplantarly elongated and are slightly concave (Fig.
6 m-o). The medial face of the bone shows a concave articular surface for the cuboid in the proximoplantar border.

The ectocuneiform of *Am. leptonhynchus* is a dorsoplantarly elongated bone, relatively proximodistally large compared with the compared canids, *Na. nasua* and *T. ornatus*. The proximal surface, which articulates with the navicular, is quadratic, slightly dorsally wider, and concave, similar to that seen in *Ai. fulgens*. The distal surface is T-shaped and articulates with the Mt III. The plantar face shows a smooth and spherical tubercle strongly protruding plantarly, similar to that of *Ai. fulgens*, *Ma. foina*, *Na. nasua*, *Po. flavus*, and *Pr. lotor* (Fig. 6p-s). This tubercle is not as developed and spherical in the other compared carnivorans. It represents the proximomedial wall of the osseofibrous plantar tunnel of the pes, as described in *Ai. fulgens* (Fisher et al. 2008). The proximal margin of the lateral face of the ectocuneiform of *Am. leptonhynchus* is marked by a dorsoplantarly wide articular surface for the cuboid, as developed as in *Na. nasua*, *Po. flavus* and *Pr. lotor* (Fig. 6p-s). This facet is not continuous in the compared canids and extends distally in *Ga. vittata*, *Gu. gulo*, *Ma. foina*, and *Me. meles*. On the distodorsal corner, there is another small and marked articular surface for the cuboid (Fig. 6p-s). The medial face has a small and triangular facet on the proximoplantar border for the mesocuneiform, which is smaller and smoother in the compared canids, *Ge. genetta*, *Ma. foina*, and *Me. meles*, while it is larger in *Ga. vittata*, *Gu. gulo*, and *T. ornatus* (Fig. 6p-s). There are also two elliptic and elongated facets on the distal border of this face for the articulation with the Mt II (Fig. 6p-s). These latter facets are well marked, like those in *Ai. fulgens*, *Ga. vittata*, *Ma. foina*, *Po. flavus*, *Pr. lotor*, and *T. ornatus*.

## Metatarsals

The Mt I is the shortest of the metatarsals of *Am. leptonhynchus*, representing two-thirds of the length of the longest metatarsal, the Mt IV (Fig. 7a). The proximal base exhibits a convex and large facet, mediolaterally wide, that articulates with the entocuneiform. On its medial side, there is a large tubercle showing an elliptical and flat surface (Fig. 7a). The plantar face of the base shows a groove for the passage and insertion of the tendon of the m. fibularis longus (Fisher et al. 2008; Ercoli et al. 2013; Liu et al. 2016). Its corpus is slightly curved, as in *Ai. fulgens*, *B. astutus* and *Po. flavus*, with a laterally tilting head (Fig. 7). As in its other metatarsals, in *Am. leptonhynchus*, the Mt I shows a bulbous head with a smooth plantarly protruding central ridge.

The Mt II, III, and IV are similar in shape. The Mt II is shorter than the Mt V. The base is triangular, dorsally larger, with a small lateral notch. Medially, there is a rugosity for the articulation with the Mt I, and laterally, two circular dorsal and plantar facets for the articulation with the Mt III. The corpus of the Mt II is slightly medially curved. The Mt III is slightly shorter than the Mt IV, with a straight corpus. Its proximal articulation with the ectocuneiform is dorsoplantarly elongated, T-shaped, with two marked lateral and medial notches. The medial face shows a rough concave surface for the articulation with the Mt II, and the lateral face has
a plantar circular facet and a small dorsal projection that covers the dorsomedial part of the Mt IV base.

In *Am. leptorhynchus*, the Mt III is similar in robustness to that of *Ga. vittata* and *Po. flavus* (MT3R = 0.11), more gracile than that of *B. astutus*, *Gu. gulo*, *Me. meles*, *N. nasua* and *T. ornatus* (MT3R between 0.12 and 0.14), and more robust than that of the compared canids, *Ai. fulgens*, *Ge. genetta* and *Ma. foina* (MT3R between 0.08 and 0.10) (Table 5). The ratio between the length of the Mt III relative and the hindlimb length (MT3L) in *Am. leptorhynchus* is similar to that of *Ai. fulgens*, *B. astutus*, *Ga. vittata*, *N. binotata*, and *Po. flavus* (0.14–0.15), while it is greater in the compared canids, *Ge. genetta*, *Gu. gulo*, and *Ma. foina* (0.16–0.19) and lesser in *Me. meles*, *Na. nasua* and *T. ornatus* (0.10–0.13) (Table 5).

The Mt IV is the longest of the five metatarsals of *Am. leptorhynchus*. The corpus is straight, slightly medially displaced with respect to the proximal articulation. This latter has a laterally concave margin. There is a medial tubercle that articulates with the Mt III, and a lateral notch for the Mt V. The dorsal border of the base slightly protrudes laterally over the Mt V proximal articular surface when articulated.

The Mt V is a long metatarsal, with a tubercle that laterally protrudes as in *Po. flavus*, on the proximal articulation (Fig. 7a). As in *Po. flavus*, the proximal articular facet is concave and larger than that of *Ai. fulgens*.

## Phalanges

The deep central groove located on extremities of the proximal and middle phalanges is due to the passage of the strong distal tendons of the m. flexor digitorum profundus (Fig. 8) (Fisher et al. 2008; Ercoli et al. 2013; Liu et al. 2016). The proximal and middle phalanges of the foot of *Am. leptorhynchus* are very similar to those of its hand (Gardin et al. 2021). Nevertheless, the formers are a little more robust and longer. The proximal phalanx of the first digit of the foot of *Am. leptorhynchus* is unknown as it was not found in articulation with a Mt I, thus it cannot be distinguished from the homologous phalanx of the hand. The proximal phalanx represents the half or the Mt III length, being long, with wide extremities. In *Am. leptorhynchus*, the ratio between the lengths of both the proximal phalanx and the Mt III (MTP) is the same as in *Na. nasua*, being greater than in the compared canids, *Ge. genetta*, *Ma. foina* and *Me. meles* (0.23–0.41) and lesser than in *Ai. fulgens*, *B. astutus*, *N. binotata*, *Po. flavus*, and *T. ornatus* (0.49–0.60) (Table 5). The middle phalanges of *Am. leptorhynchus* are shorter and slenderer than the proximal ones and have a straight shaft. As for the middle phalanges of the hand, the middle phalanges of the foot show an asymmetrical head, as seen in *Ai. fulgens*, *G. genetta*, *Po. flavus*, and *N. binotata* (Fig. 8).

The distal phalanges of the hindlimb of *Am. leptorhynchus*, found in anatomical connection with the foot, display a size similar to that found in anatomical connection with the hand and are very similar in shape (Gardin et al. 2021). Its distal phalanges are mediolaterally narrow,
very high dorsoplantarly and craniocaudally short, being very similar to those of small climbing
carnivorans (Fig. 8).

**2. PCA of the hindlimb morpho-functional indices**

The first two principal component axes of the analysis on the morpho-functional indices of the
hindlimb account for 64.4% of the total variation (Fig. 9). Towards the positive side of PC1 are
located those forms with high values in CRW, CMT, and CEFL and low values for MT3L and
CI, indicating that from the negative to the positive side of the axis, the compared species tend
to show a developed fibular tubercle on the calcaneus, longer calcaneal ectal facet, and shorter
distal elements of the hindlimb (tibia and metatarsals). Towards the positive side of PC2 are
located the taxa with high values for IRD, GLU, FCP, THI, and CTC, and low values for FHI:
from the negative to the positive side of the axis, the species tend to display a proximally
extended femoral greater trochanter, caudally protruding femoral condyles, a craniocaudally
wide tibial proximal epiphysis, a long tuber calcanei, and a craniocaudally wide femoral head.
Although the canids constitute a cluster, the rest of the studied taxa are mixed in the
morphospace totally independantly from the phylogeny, as pictured by the crossing lines of the
phylogenetic projection.

The overall distribution of the different taxa in the morphospace shows the most of the climbing
carnivorans in the negtive part of PC2, the canids constitute a cluster in the negative part of
PC1 and positive part of PC2, while *Gu. gulo*, *Me. meles*, *Na. nasua*, and *T. ornatus* are located
in the positive part of PC1 and PC2 (Fig. 9). In this morphospace, *Am. leptorhynchus* plots near
the scansorial *B. astutus*, the tree-dwellers *Ai. fulgens* and *N. binotata* and the terrestrial
generalist *Ga. vittata*, being located in the positive part of PC1 and negative part of PC2. The
position of *Am. leptorhynchus* in this morphospace highlights its hindlimb proportions
resemblance with those of *Ai. fulgens*, *B. astutus*, *Ga. vittata*, and *N. binotata*.

**3. Hindfoot reversal categories discrimination and attribution of *Am. leptorhynchus***

The first 2 principal component axes of the analysis on the ratios on the ankle articulation account for 43.9% of the total variation (Fig. 10). On the one hand, towards the positive side of PC1 are located those forms with high values in T_LDE, C_LPT, C_LapST and A_HmaxCT and low values in C_DPFCT, indicating that from the negative to positive side of the axis, the studied species tend to show relatively mediolaterally wide distal articulation of the tibia, a proximally longer fibular tubercle, a craniocaudally longer sustentaculum tali, a higher talar trochlea and an elongated calcaneal ectal facet. In the most negative part of the PC1 are located the non-reversers C. familiaris. Most of the fullreversers plot in the negative to slightly positive part of the PC1, except *Ai. fulgens* specimens, which are in the positive part of this axis. The partial-reversers are located around zero. On the other hand, towards the positive side of PC2 are located the taxa with high values in A_MaxWH and A_WCE, and low values in T_LMM, C_WmlST, showing that from the negative to positive side of the axis, the studied species tend
to be characterized by a relatively mediolaterally wider talar head, a wider caudal face of the talus, a shorter medial malleolus of the tibia, and a mediolaterally shorter sustentaculum tali. The majority of the non- and partial-reversers are found in the positive part of the PC2, while most of the full-reversers (except *Ai. fulgens*) are located in the negative part of PC2. In this morphospace, the estimated average individual of *Am. leptorhynchus*, represented by the means for each ratio on all the specimens studied, plots in the positive part of PC1 and in the negative part of PC2. It is very close to a specimen of *Pr. lotor*, which is very isolated from other specimens of its species. In this region of the morphospace, we find mostly full-reversers specimens and some isolated partial-reversers, indicating their morphological similarity for the ankle articulation with the small fossil arctoid.

The results of the leave-one-out cross-validated linear discriminant analyses are given in Table 6; Fig. 11. After the LDA results for the tibia, none the of non-reversers specimens, 74% of the partial-reversers and 68% of the fullreversers were well classified (accuracy 0.66, Kappa index 0.38). The results of this analysis attribute *Am. leptorhynchus* to the full-reverser category with a probability of 100% for all its measured tibiae, meaning that the tibial end of *Am. leptorhynchus* is like that of full-reversers (Fig. 11). The results obtained after the LDA for the talus shows that 67% of non-reversers specimens, 77% of the partialreverser specimen, and finally 83% of the full-reverser specimens were correctly attributed to their respective groups (accuracy 0.79, Kappa index 0.63). The results of the LDA performed on the talar shape classify 80% of the *Am. leptorhynchus* tali in the partial-reversers category, and 20% in the full-reversers, indicating that its talus is morphologically like that of the partial-reversing specimens (Fig. 11). For the calcaneus, correct classification results for the cross-validated analyses were 100% for non-reversers, 59% for partial-reversers, and 72% for full-reversers (accuracy 0.69, Kappa index 0.46). The results of the LDA predictions using the calcaneal shape data attribute *Am. leptorhynchus* to the full-reversers at a probability of 100%, suggesting that its calcaneus is very similar to that of the full-reversers (Fig. 11). Finally, the LDA based on the whole ankle morphology (from the first 2 PCs) allowed to correctly classify 83% of non-reversers, 77% of partial-reversers and 76% of full-reversers (accuracy 0.77, Kappa index 0.60). The results of this analysis attributed the average individual of *Am. leptorhynchus* to the full-reverser category (79.4%) and to the partial-reverser category (20.5%). This fictive average morphology plots near a *B. astutus* and *Po. flavus* specimens in the morphospace (Fig. 11).

Non-reversers are often well classified thanks to our models, except that based on the tibia. The distinction between partial- and full-reversers is more complicated, but the best predictions for these two categories are obtained through measurements made on the talus as well as when the three bones are brought together to create the model.

# Discussion

## 1. Morphofunctional Implications

### Coxo-femoral articulation
The coxal bone and the femur, especially the trochanters, are both partly modelled by muscular actions of the hindlimb, which is involved in the support and propulsion of the body (Davis 1964; Argot 2003; Polly 2007). There are two groups of muscles involved in the extension of the hip: the gluteal complex and the ischiopubic complex. The relative depth of the ilium (IRD) and the proximal extension of the greater trochanter (GLU) revealed to be more informative, indicating the relative development and lever arms of the gluteal muscles. Cursorial and semi-fossorial species show greater values in IRD and GLU, corresponding to a more developed gluteal complex (mm. glutei), related to a fast extension of the hip. Moreover, some species with strong epaxial musculature, such as *Gu. gulo* or the giant panda, *Ailuropoda melanoleuca*, show a wide and laterally flaring ilium, representing an advantage for the leverage of the gluteal muscles in both erected and burrowing postures, as it increases stability (Davis 1964; Argot 2003). The long femoral neck and the low greater trochanter observed in *Ga. vittata* and *Me. meles* indicate a lateralised and adducted position of the femur, with the hip articulation more stable but less mobile than observed in climbing carnivorans (Jenkins and Camazine 1977; Ercoli 2015). On the contrary, the narrow and parasagittally oriented ilium and short greater trochanter of *Am. leptorhynchus*, shared by most of the studied climbing carnivorans and *Ga. vittata*, indicate a lesser surface available for the attachment of the gluteal muscles (also involved in hip abduction in this configuration), which benefits the ischiopubic complex. Indeed, the elongated ischium, with a moderately developed ischial tuberosity, and the distally extending cranial tuberosity of the tibia of *Am. leptorhynchus* indicate the greater development and greater moment arms of the hamstring muscles (strong hip extensors), relative to the gluteal group (weak hip extensors), as seen in terrestrial and climbing half-bounders, such as *B. astutus*, *Ga. cuja*, *Ma. foina*, and *N. binotata* (Ercoli et al. 2013; Ercoli and Youlatos 2016; Liu et al. 2016) contrary to slow climbers (e.g., *Po. flavus*, Marsh et al. 2021) or ambulatorial carnivorans (e.g., *Gu. gulo*, Ercoli et al. 2013). Moreover, the parasagittally oriented ilium increases the moment arm of the extensors of the lumbar region (the m. iliocostalis lumborum). This configuration of the hip articulation favours a slower and more powerful hip extension, on a parasagittal plane, allowing to propel the body while bounding, jumping, or climbing on a vertical support (Smith and Savage 1956; Taylor 1976; Argot 2002; Ercoli et al. 2013; Ercoli and Youlatos 2016).

On the other hand, the configuration of the gluteal muscles in *Am. leptorhynchus* indicates that those muscles were more involved in hip abduction rather than in hip extension (Taylor 1976). The location of the ischial spine of *Am. leptorhynchus*, caudal to the acetabulum, and its great development are typical of skilful climbing carnivorans (Gebo and Rose 1993; Ercoli and Youlatos 2016). This structure serves for the origins and passage of the lateral rotators and hip abductors, m. obturatorius internus and mm. gemelli, thus increasing their leverage in this configuration and thus increasing the range of rotation of the femur, as described in other carnivorans (Heinrich and Houde 2006; Zack and Rose 2015; Salesa et al. 2019). The acetabulum and the femoral head of *Am. leptorhynchus* are typical of climbing or ambulatory carnivorans: the former is shallow and widely opened and the latter is spherical (FHI = 1)
oriented proximally and with the dorsal border gently continuing onto the neck. These features permit great mobility of the hip articulation, particularly in terms of rotation and abduction, being useful for climbing or walking on an uneven substrate (Jenkins and Camazine 1977; Gebo and Rose 1993; Argot 2003; Heinrich and Houde 2006; Polly 2007; Ercoli and Youlatos 2016). The large and mediocaudally protruding lesser trochanter of *Am. leptorhynchus*, providing the insertion areas for the m. iliopsoas and the m. quadratus femoris (hip rotators), highlights the great range of non-parasagittal movement of its hip articulation making it easier to place its foot facing towards the substrate when climbing (Taylor 1976; Argot 2003; Makungu et al. 2014; Ercoli and Youlatos 2016). However, the lesser trochanter is slightly less medially oriented in *Am. leptorhynchus* than in *Ai. fulgens*, indicating a slight parasagittal constraint of the hip movement compared to this latter taxon, as described in terrestrial and climbing bounders (Ercoli and Youlatos 2016). This configuration could indicate that the abducted posture was possible but mainly used when crouched or climbing, and not in a systematic way, as in bounding climbers such as *N. binotata* (Ercoli and Youlatos 2016).

The pubic ramus of *Am. leptorhynchus* is thick and represents the origin area of the main adductors of the hindlimb that allow increasing the ability to grip the substrate and give power for climbing (Taylor 1976; Makungu et al. 2014).

## Knee articulation

The knee articulation of *Am. leptorhynchus* shares many similarities with that of half-bounding terrestrial or climbing carnivorans, such as *B. astutus*, *Ga. vittata*, *Ge. genetta*, and *N. binotata*. The relatively wide femoral trochlea of *Am. leptorhynchus* (but less than in *Po. flavus*), delimitated by low ridges, the wide tibial plateau, the relatively flat tibial tuberosity and the marked scar for the medial collateral ligament on the tibial shaft indicate knee movement without parasagittal constraint, stabilised by soft tissues, and a relatively crouched locomotion (Ginsburg 1961; Carrano 1997; Argot 2003; Ercoli 2015; Ercoli and Youlatos 2016). However, this posture is slightly contradicted by the height of the femoral trochlea of *Am. leptorhynchus*, being higher than in slow climbing carnivorans and the moderate caudal projection of the femoral condyles, as an intermediate between the terrestrial and the climbing carnivorans, which suggest a greater excursion of the patella and thus a slightly more extended posture for this fossil arctoid (Ginsburg 1961; Carrano 1997; Argot 2003).

The salient tubercle cranial to the acetabulum in *Am. leptorhynchus* indicates well-developed m. rectus femoris (part of the m. quadriceps femoris). The m. quadriceps femoris is a strong extensor of the knee, and is well-developed in terrestrial and cursorial taxa, also suggested by their cranially protruding tibial tuberosity. However, some climbing carnivorans also have a highly developed m. rectus femoris, as evidenced by the salient tubercle cranial to the acetabulum in *N. binotata* for example, like in *Am. leptorhynchus*. In this configuration, the m. quadriceps femoris acts as a powerful knee extensor (rather than a fast extensor as in terrestrial and cursorial taxa), thus allowing jumping, as well as fast and agile climbing activities in extant
mammals (Taylor 1976; Jungers et al. 1980). In contrast, slow and cautious climbers do not exhibit a well-developed m. quadriceps femoris (e.g., less developed in *Po. flavus* than in *B. astutus*; see Liu et al. 2016; Marsh et al. 2021). Demes et al. (1998) show that leaping specialist primates have a relatively more developed m. quadriceps femoris than generalists primates. They also hypothesise that the hamstring muscles and m. gastrocnemius might help in transferring the power from knee to hip and ankle while jumping, or they might exert a flexion when the knee is getting extended.

## Hindfoot anatomy

The articular surfaces located on the distal epiphysis of the fibula of *Am. leptorhynchus* are wide, as noted in the climbing carnivorans, and contrary to the cursorial species, in which these surfaces are much narrower. This configuration may be an adaptation of climbing species, with a mobile well-separated fibula during plantarflexion or dorsiflexion, for moving on uneven surfaces, on arboreal or rocky substrates, and to a plantigrade posture (Taylor 1976; Gebo and Rose 1993; Argot 2003). The obliquity of the plantar groove on the talar trochlea of *Am. leptorhynchus*, as well as its large plantar tubercle on the cuboid, the short metatarsals, and long proximal phalanges relative to metatarsals also indicate its plantigrade stance (Gebo and Rose 1993; Wang 1993). More precisely it indicates its sub-plantigrade stance because, in extant carnivorans, the proximal extremity of the calcaneus does not meet the ground during the stance phase or while climbing (Gebo and Rose 1993).

The robustness of the tuber calcanei in *Am. leptorhynchus* might provide a mechanical advantage and increase the insertion area for the tendo calcaneus, which is composed in part by the m. gastrocnemius, as described in mustelids (Ercoli 2015). In this early arctoid, the lateral head of this muscle might originate from a tubercle proximal to the lateral femoral epicondyle which is visible in few of the compared taxa (canids, *Ge. genetta*, *Na. nasua* and *Po. flavus*). The m. gastrocnemius is a knee flexor and is also involved in powerful ankle extension, for fast acceleration as required for jumping, bounding, galloping and swimming (Walmsley et al. 1978; Fisher et al. 2008; Ercoli 2015; Morales et al. 2018). The presence (e.g., in *Po. flavus*) or absence (e.g., in *B. astutus*) of the tubercle proximal to the lateral femoral epicondyle does not seem to indicate the mass of this muscle: the m. gastrocnemius and m. flexor digitorum superficialis (the main ankle extensors) as well as the m. biceps femoris (as part of the hamstring muscles) are more developed in *Ga. cuja* and *B. astutus*, agile predators, than in *Po. flavus*, slow climber (Ercoli et al. 2013; Liu et al. 2016; Marsh et al. 2021). However, its proximal origin in *Am. leptorhynchus*, probably through a strong tendinous attachment, might result in a longer thus stronger m. gastrocnemius than in the other carnivorans having a more distal origin area. Then, more than rapid movements as for cursorial carnivorans which have a more developed gluteal group, powerful hind limb extension movements are favoured, such as rapid accelerations and jumps, by the configuration seen in *Am. leptorhynchus* (Ercoli et al. 2013).
A key adaptation to climbing can be highlighted when studying the ankle morphology: the hindfoot reversal depicts the ability to reverse the foot 180°, in such a manner that the plantar surface of the foot lies in a medial or dorsal position (Jenkins and McClearn 1984). Hindfoot reversal is known in various groups of extant mammals with arboreal adaptations, especially carnivorans such as *Ai. fulgens*, *B. astutus*, *Leopardus wiedii*, *Ma. foina*, *N. binotata*, and *Po. flavus* (Taylor 1970; Trapp 1972; Jenkins and McClearn 1984; Meldrum et al. 1997; Makungu et al. 2014; Liu et al. 2016; Morales et al. 2018) and has also been inferred in the fossil hyaenodontid genus Arfia (Gingerich and Deutsch 1989). However, many other extant species can perform a partial reversal (90° reversion), like the other procyonids, or as inferred for the fossil hyaenodontid Prolimnocyon atavus (Trapp 1972; Jenkins and McClearn 1984; McClearn 1992; Gebo and Rose 1993; Makungu et al. 2014).

Some authors have taken a deep interest in the myological and osteological features associated with hindfoot reversal. For instance, this ability appears to lie on osteological features rather than on myological ones in *B. astutus* (probably related to its small body size relative to the other compared taxa), especially on the morphology of the articular surfaces composing the crurotalar and subtalar transverse tarsal articulations, while in *Po. flavus* and *L. wiedii* both osteological and myological features have revealed to be adapted to hindfoot reversal (Trapp 1972; Jenkins and McClearn 1984; Liu et al. 2016; Morales et al. 2018; Marsh et al. 2021).

According to Jenkins and McClearn (1984), who explored the different possibilities to perform the hindfoot reversal in mammals, this ability is mainly based on the movement of plantarflexion at the crurotalar articulation, inversion at the subtalar articulation and supination at the transverse tarsal articulation. In *Po. flavus*, Jenkins and McClearn (1984) described that during hindfoot inversion, the calcaneus rotates medially and shifts cranially in relation to the talus. Thus, the sustentaculum tali shifts from a plantar position, under the talar head, to a more medial position between the talar head and the medial malleolus of the tibia. The cuboid facet of the calcaneus lies in a plantar position in relation to the talar head, from a more plantar and proximal position.

As described for carnivorans able to fully rotate their foot, *Am. leptonhynchus* shows smooth, continuously slightly curved articular facets between the talus and calcaneus, a feature that allows for greater mobility of the ankle articulation (Taylor 1976; Makungu et al. 2014). Meldrum et al. (1997) characterized the osteological features allowing the hindfoot reversal and increasing the ankle mobility, inversion and eversion abilities. Many of them are found in *Am. leptonhynchus*, such as the development of the medial malleolus (as in *Ai. fulgens* and *Po. flavus*), the extension of the talar sustentacular facet on the plantar surface of the talar neck (as observed in *Ai. fulgens*, *B. astutus* and *Po. flavus*), the different orientation of the calcaneal ectal facet and the sustentaculum tali, the medially oriented talar neck, and the large bulbous talar head which articulated with the deeply concave proximal surface of the navicular, typical of climbing carnivorans (Taylor 1976; Meldrum et al. 1997; Chester et al. 2015; Ercoli and Youlatos 2016; Panciroli et al. 2017; Marsh et al. 2021). In comparison, *Na. nasua*, partial-
reverser, shows a flatter sustentacular facet that does not medially extend enough, its shorter
and wider talar neck, thus inhibiting full inversion of its hindfoot (Meldrum et al. 1997).
Moreover, the calcaneal ectal facet is very important during the inversion, as the calcaneus
shifts cranially; its elongated (high value in CEFL, as in *N. binotata* and *Po. flavus*), convex
and continuous shape in *Am. leptorhynchus*, more than in *Na. nasua*, allows the talus to
articulate with the proximal part of the articular surface when walking on a horizontal support,
and going on the caudal part during inversion of the foot, as it is described for *Po. flavus*
(Jenkins and McClearn 1984). In carnivorous able to reverse their hindfoot, a greater talar
trochlea curvature was expected and is considered an important feature (Trapp 1972; Jenkins
and McClearn 1984; Meldrum et al. 1997; Liu et al. 2016). *Am. leptorhynchus* has a mean talar
curvature (ACV) greater than that of *Ai. fulgens*, *B. astutus*, *Ma. foina*, *N. binotata*, and *Na.
nasua* (partial-reverser) and lower than that of *Po. flavus*. The recent study by Marsh et al.
(2021) demonstrated that this feature varies a lot between species, whether or not they can rotate
their foot. Contrary to *Me. meles*, *Am. leptorhynchus* does not exhibit a talar foramen, thus
allowing for further plantarflexion. However, the range of ankle extension of *Am. leptorhynchus*
is lesser than that of cursorial carnivores, as the shift of the tibia on the talar trochlea is limited
caudally by a tubercle at the caudal end of the medial lip of the trochlea and the obliquity of the
tendinal groove of the caudal end of the talar trochlea, that blocks the tibia (Wang 1993). On
one hand, the talar trochlea extending smoothly onto the neck and, on the other hand, the
asymmetry of the crurotalar articulation (obliquity of the smooth crest on the distal tibia and
the asymmetrical talar trochlea) make the feet passively abduct and evert during dorsiflexion or
invert during plantarflexion, as well as allowing a greater mediolateral movement of the foot
(Taylor 1976; Jenkins and McClearn 1984; Gebo and Rose 1993; Heinrich and Rose 1997;
Meldrum et al. 1997; Heinrich and Houde 2006; Makungu et al. 2014; Chester et al. 2015). The
predictions of the LDAs for *Am. leptorhynchus* preferentially place it in the category of full-
reversers according to measurements taken on the tibia and calcaneus, or rather in partial-
reversers, according to measurements on talus. The entire dataset places *Am. leptorhynchus*
(based on a composed individual taken from the averages of all ratios studied) in the category
of full-reversers, indicating that the overall shape of its ankle is similar to that of full-reversers
(Fig. 11). These results support the idea of a very mobile ankle in *Am. leptorhynchus*, which
was therefore very likely able to reverse its foot, maybe up to 180°, as indicated by other
osteological characters.

From a myological point of view, the muscles involved in foot inversion (m. tibialis cranialis
and m. tibialis caudalis), eversion and plantarflexion (m. fibularis longus and m. fibularis
brevis) were probably important in *Am. leptorhynchus*, given the strongly marked attachment
structures for those muscles: the deep and distally extending depression on the lateral face of
the tibia for the origin of the m. tibialis cranialis; the prominent ridges delimiting the origin area
on the tibia and fibula for the m. tibialis caudalis, the marked passage for its tendon on the distal
epiphysis of the tibia and at the level of the large plantar tubercles on the cuboid and
ectocuneiform, the large cranial tuberosity of the fibular head for the origin of the m. fibularis
longus, its deep distal groove and the deep plantar groove on the cuboid for the passage of its
tendon on the fibula. The development of those muscles would have been linked to the hindfoot
reversal ability of *Am. leptorhynchus*, as it has been suggested in extant mammals with this
capacity (Jenkins and McClearn 1984; Morales et al. 2018; Marsh et al. 2021). The m. tibialis
caudalis and the m. tibialis cranialis produce a strong inwards inversion of the foot, which can
allow to turn the foot inwards while climbing (Salesa et al. 2008; Morales et al. 2018).
Moreover, the larger, laterally protruding and proximally placed fibular tubercle of the
calcaneus would increase the leverage of m. fibularis longus and m. fibularis brevis that pass
caudal to it, allowing to counterbalance the forces displayed for inversion during the foot
reversal (Chester et al. 2015).

Besides, if *Am. leptorhynchus* was able to reverse its feet, therefore it was probably able to
perform hindlimb suspension, which requires strong digital flexor muscle to support the body
weight against gravity and stay firmly grasped on the support, upside down (Liu et al. 2016).
On the other hand, it is interesting to note that *Am. leptorhynchus* is smaller than the species
able to perform suspension mentioned in this article, except *B. astutus*. Its small body size could
exempt it from having extremely developed digital flexors (Morales et al. 2018). But as is also
the case for *B. astutus* (Liu et al. 2016), the morphology of the hind limb indicates that the
digital flexors of *Am. leptorhynchus* were powerful. Indeed, the large origin area extending
distally on the diaphysis in addition to the strong medial tubercle on the proximal epiphysis of
the fibula of *Am. leptorhynchus* suggest a well-developed m. flexor digitorum medialis, as
described in arboreal carnivorans with foot inversion and suspension abilities (Fisher et al.
2008; Liu et al. 2016; Morales et al. 2018). When articulated together, the tibia and the fibula
form a large space, larger than in cursorial taxa, allowing a more digital flexor musculature,
very important to flex the digits in arboreal taxa (Argot 2002). The talar tendinal groove, located
caudally to the talar trochlea, provides a passage for the m. flexor digitorum lateralis. This
muscle is involved in very important movements for arboreal mammals when grasping a
support, as it permits flexion of the fingers and plantarflexion of the hindfoot (Chester et al.
2015; Liu et al. 2016). The development of this groove indicates that *Am. leptorhynchus* was
probably able to perform hindlimb suspension and resist gravity thanks to its well-developed
digital flexors. The fibular tubercle on the calcaneus also serves as the origin area of the m.
quadratus plantae, thus increasing its leverage and its utility as a digital flexor during hind limb
suspension (Chester et al. 2015; Liu et al. 2016; Panciroli et al. 2017).

In the same way as for arboreal carnivorans, the metatarsals of *Am. leptorhynchus* are robust,
short, and relatively fan-shaped placed, unlike those of cursorial carnivorans (Argot 2003). Its
widely spaced metatarsals allow the development of powerful muscles allowing great mobility
of the digits (Makungu et al. 2014). The Mt I of *Am. leptorhynchus* and arboreal carnivorans is
well-developed, contrary to that of cursorial species again, in which it is reduced or missing
(Taylor 1976). The wide base of the Mt I of *Am. leptorhynchus*, as well as its groove located
palmarly, serves for the insertion of the m. fibularis longus and m. tibialis cranialis (Fisher et
al. 2008; Liu et al. 2016), thus indicating great grasping abilities by having a divergent first digit and possibly opposable digits (Taylor 1976; Argot 2002; Salesa et al. 2008). In the Mt IV of *Am. leptorhynchus*, the articular facet for the Mt III is relatively distally located on its corpus. Salesa et al. (2008) highlighted this characteristic in *Ai. fulgens* and *Po. flavus*, thus indicating greater mobility between the third and fourth digits. The Mt V shows a strong lateral projection on the base, where both the m. fibularis brevis and m. abductor digiti V attach (Fisher et al. 2008; Liu et al. 2016). Typical of arboreal carnivorans, this projection indicates a greater ability to abduct the fifth digit and to evert the foot, in *Am. leptorhynchus* (Taylor 1976; Salesa et al. 2008, 2013; Makungu et al. 2014). As noticed for the phalanges of its hand, the asymmetry of the head of the middle phalanges of the foot of *Am. leptorhynchus* indicates that its claws were semi-retractile (Taylor 1974), thus allowing better grasping abilities, by controlling the movement of the claws. Indeed, the asymmetry allowed the distal phalanx to retract itself, near the middle phalanx, as described by Taylor (1976), for *N. binotata* and *Ge. genetta*.

**Summary of the Inferences on the Functional Morphology of the Hindlimb**

The study of the forelimb of *Am. leptorhynchus* had already shown that this carnivoran had clear adaptations to climbing (Gardin et al. 2021). Based on the analysis of the functional anatomy of the hind limb of *Am. leptorhynchus*, it is possible to go further in the understanding of its locomotor capacities, which are for the majority linked to an excellent ability to climb. The hip articulation of *Am. leptorhynchus* is mainly characterised by its great range of mobility, thanks to the shape of its articular surfaces, and by the development and the increased leverage of the abductor, adductor and rotator musculature, allowing to adapt the orientation of the hindfoot and firmly grip the substrate. However, its range of hip abduction was probably slightly lower than that of slow climbers, indicating a abducted posture used mainly for walking on uneven substrate, climbing and in crouched posture, as described for bounding climbers such as *N. binotata*. Furthermore, the morphology of the knee of *Am. leptorhynchus* suggests a mobile articulation, without parasagittal constraint and stabilised by soft tissues, and showing a slightly crouched posture.

Many osteological similarities are highlighted with *Ga. vittata*, a terrestrial mustelid, stronger than expected based on the study of the forelimb. These similarities are shared with other climbing taxa, also half-bounders such as *B. astutus*, *Ge. genetta* and *N. binotata*. Indeed, the extensors of the hindlimb of *Am. leptorhynchus* were probably strong in their configuration, thus favouring powerful extension movement such as jumps and rapid accelerations, in contrast to the slow climbers with less developed extensors. This adaptation allowed this early arctoid to jump between branches, perform vertical looping which is a controlled ascent or descent on a vertical support, but also to capture preys.

By studying the morphology of its ankle, which shares many similarities with that of *Ai. fulgens*, *B. astutus*, *N. binotata*, and *Po. flavus*, it is possible to affirm the capacity of *Am. leptorhynchus* to perform a reversion of the foot, perhaps even up to 180°. This ability permitted *Am*.
*leptorhynchus* to skilfully descend vertical support headfirst whether on tree trunks or vertical rocky supports. Besides this, *Am. leptorhynchus* was also probably capable to perform hindlimb suspension thanks to its well-developed digital flexors. They allowed it to firmly grasp the support with its foot fingers, while the body was suspended, allowing the animal to reach food or another branch, and capture prey with its forepaws (Trapp 1972; Jenkins and McClearn 1984; Morales et al. 2018). Its semi-retractile claws and its long and mobile digits allowed *Am. leptorhynchus* to have a firm grasp. Like many arboreal carnivorans, the hindlimb of *Am. leptorhynchus* had a sub-plantigrade stance. It means that its heel did not come into contact with the ground during the stance phase or while climbing.

## 2. Palaeoecological remarks

The study of the limb anatomy of *Am. leptorhynchus* strongly suggests that this species developed good climbing abilities. The locomotor behaviour of this small early arctoid and its ecology are becoming clearer thanks to this new study. Contrary to the hypothesis put forward from the morphology of the forelimb which suggested that *Am. leptorhynchus* could belong to the class of slow climbers (Gardin et al. 2021), the hindlimb seems to picture a faster and more agile animal, being able to climb with control headfirst upwards and downwards, but also perform jumps and bounds between branches or to capture prey, such as the extant climbing half-bounders such as *B. astutus* or *N. binotata*. The adaptations for jumping and to bounding gait in *Am. leptorhynchus* could be supported by a future complementary study of the anatomy of its axial skeleton, focusing on the flexibility and the length of the vertebral column in particular (Ercoli and Youlatos 2016).

However climbing skills do not necessarily mean an arboreal lifestyle. Even if it is the most common lifestyle generally among small climber carnivorans, some of them also live in rocky environments, where their adaptations to climb serve them to evolve on uneven substrates (Trapp 1972; Wilson and Mittermeier 2009). However, the great diversity of small carnivorans and glirids in the Itardies locality suggests that *Am. leptorhynchus* lived in a relatively closed and forested environment (Vianey-Liaud 2003; Bonis pers. comm.). The dental morphology of *Am. leptorhynchus* suggests an omnivorous diet, a broad-spectrum diet (Cirot and Bonis 1992). Its good grasping abilities with its hand and its ability to perform hindfoot reversal and hind limb suspension probably allowed *Am. leptorhynchus* to fully exploit its spatial environment, forests or rocky areas, to access food resources more easily, such as eggs, fruits, and to use stealth and jumps to capture small preys, as does the extant African palm civet *N. binotata* (Taylor 1989; Morales et al. 2018).

This study complements the first of its kind on the functional anatomy of a member of the Amphicynodontidae, allowing to start the investigation on the ecology of a little known carnivoran family often present in the Oligocene deposits of Eurasia (Wang and Qiu 2003). This work has allowed to highlight new climbing adaptation of *Am. leptorhynchus*, especially its probable ability to fully reverse its hindfoot, and its capacity to hang by its hindlimbs, which
are rarely seen in fossil carnivorous, if never. Exploration of the ecology of members of this family of small early arctoids should also focus on their diet and the analysis of locomotor adaptations of its other members, to go further.

**Supplementary Information.** The online version contains supplementary material available at
https://doi.org/10.1007/s10914-022-09621-9.

**Acknowledgements.** We thank the reviewers for their insightful comments which allowed to improve this article. We thank all the people who worked with LdB in the locality Itardies. We are grateful to the members of the UMR 7262 (PALEVOPRIM, CNRS and University of Poitiers) and of the Museo Nacional de Ciencias Naturales-CSIC, in which this paper was prepared, and especially Camille Grohé for our constructive discussions, Jérôme Surault for taking some of the photos of the fossil specimens and Pablo Peláez-Campomanes for providing access to the microscope to take pictures of the smallest skeletal elements. We also thank Heather F. Smith for kindly providing us with the measurements protocol used for the LDA. We thank the Anatomical Collections of the Museo Nacional de Ciencias Naturales-CSIC (especially its curator, Ángel Luis Garvía), and the Museo Anatómico de la Universidad de Valladolid, the Collections of Mammals and Birds of the Muséum National d'Histoire Naturelle - Paris (especially their curators Géraldine Véron, Céline Bens and Aurélie Verguin) for providing the large sample of skeletons of extant carnivorous used for comparison.

**Authors' Contributions.** Louis de Bonis and Manuel J. Salesa designed the study. Axelle Gardin made the descriptions, analysis, and interpretation of the anatomy of the hindlimb of *Am. leutorhynchus* and draft the manuscript. All the authors revised and improved this manuscript.

**Funding.** The research was funded by the French CNRS in the frame of the "Recherche coopérative sur programme : Phosphorites du Quercy". This work is part of the PhD thesis of AG, who received financial assistance from the UMR 7262 (PALEVOPRIM, CNRS and University of Poitiers), the ASAP project (project n°210724, Nouvelle-Aquitaine region, France, PI: Olga Otero) and the Ministère de l'Enseignement supérieur, de la Recherche et de l'Innovation (France). This study is also part of the research project PID2020-112642GB-I00 funded by MCIN/AEI/ https://doi.org/10.13039/501100011033. Manuel J. Salesa is member of the Research Groups CSIC 641538 (Museo Nacional de Ciencias Naturales-CSIC, Madrid, Spain), and FOCONTUR (Fundación Conjunto Paleontológico de Teruel-Dinópolis, Teruel, Spain).

**Data Availability.** All data generated or analysed during this study are included in this published article and its Supplementary Information files.

**Conflict of Interest.** The authors declare no conflict of interest.
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Figures

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Coxal bone. a. *Am. leptorhynchus* ITD-VF-10242; b. *Ai. fulgens*; c. *V. vulpes*; d. *Me. meles*; e. *B. astutus*; f. attachment areas of the main muscles of the coxal bone of *Am. leptorhynchus*; a-f. left coxal bone in lateral view. Abbreviations: ac, acetabulum; bi, body of the ilium; dcrs, dorsocranial spine; dcs, dorsocaudal spine; il, iliac wing; is, ischiatic spine; isc, ischium; it, ischial tuberosity; of, obturator foramen; ps, pelvic symphysis; pu, pubis; trf, tubercle for the origin area of the m. rectus femoris; vcs, ventrocaudal spine. Scale bars equal 1 cm</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Femur. a. *Am. leptorhynchus* ITD-VF-10001; b. *Ai. fulgens*; c. *V. vulpes*; d. *Me. meles*; e. *B. astutus*; f. attachment areas of the main muscles of the femur of *Am. leptorhynchus*; a-f. left femur in cranial (first row), caudal (second row), lateral (third row) and distal (fourth row) views. Abbreviations: ap, articular surface for the patella; fa, facies aspera; fh, femoral head; fn, femoral neck; gt, greater trochanter; if, intercondylar fossa; lc, lateral condyle; llfa, lateral lip of the facies aspera; lt, lesser trochanter; mc, medial condyle; mlfa, medial lip of the facies aspera. Scale bars equal 1 cm</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Tibia. a. *Am. leptorhynchus* ITD-VF-10594a; b. *Ai. fulgens*; c. *V. vulpes*; d. *Me. meles*; e. *B. astutus*; f. attachment areas of the main muscles of the tibia of *Am. leptorhynchus*; a-f. left tibia in cranial (above), caudal (middle), and proximal (below) views. Abbreviations: af, proximal articular surface for the fibula; at, articular surface for the talus; fn, fibular notch; gtc, groove for the tendon of the m. tibialis caudalis; ie, intercondylar eminence; lc, lateral condyle; mc, medial condyle; mm, medial tibial malleolus; tc, tibial crest; tt, tibial tuberosity. Scale bars equal 1 cm</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Fibula. a. *Am. leptorhynchus* ITD-VF-10594b; b. *Ai. fulgens*; c. *V. vulpes*; d. *Me. meles*; e. *B. astutus*; f. attachment areas of the main muscles of the fibula of *Am. leptorhynchus*; a-f. left tibia in medial (above), and lateral (below) views. Abbreviations: at, articular surface for the talus; ati, proximal articular surface for the tibia; cat, caudal proximal tubercle; crt, cranial proximal tubercle; fh, fibular head; gf, groove for the tendon of the m. fibularis brevis and for the m. fibularis longus; lm, lateral fibular malleolus; mt, medial proximal tubercle; tn, tibial notch. Scale bars equal 1 cm</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Talus and calcaneus. a, e. *Am. leptorhynchus* ITD-VF-10863 and ITD-VF-10148; b, f. *Ai. fulgens*; c, g. *V. vulpes*; d, h. *Me. meles*; a-d. left talus in dorsal (above) and plantar (below) views, e-h. left calcaneus in dorsal (above) and medial (below) views. Abbreviations: cef, posterior calcaneal ectal facet; cu, articular surface for the cuboid; eft, talar ectal facet; fdl, groove for the m. flexor digitorum lateralis; ft, fibular tubercle; gmt, groove for the medial malleolus of the tibia; na, articular surface for the navicular; pct, plantocaudal tubercle; sfc, calcaneal sustentacular facet; sft, talar sustentacular facet; st, talar sulcus; sta, sustentaculum tali; tc, tuber calcanei; th, talar head; tn, talar neck; tt, talar trochlea. Scale bars equal 0.5 cm</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Navicular, cuboid and cuneiforms. a, e, i, m, p. *Am. leptorhynchus* ITD-VF-11107, ITD-VF-10202, VF-12293, ITD-VF-12294 and ITDVF-10126; b, f, j, n, q. *Ai. fulgens*; c, g, k, r. *V. vulpes*; d, h, l, o, s. *Me. meles*; a-d. left navicular in proximal (above) and distal (below) views; e-h. left cuboid in plantar (above) and medial (below) views; i-l. left entocuneiform in lateral view; m-o. left mesocuneiform in medial view; p-s. left ectocuneiform in lateral (above) and medial (below) views. Abbreviations: ca, articular surface for the calcaneus; cu, articular surface for the cuboid; ec, articular surface for the ectocuneiform; en, articular surface for the entocuneiform; gfl, goove for the tendon of the m. fibularis longus; ltp, lateral plantar tubercle; me, articular surface for the mesocuneiform; mpt, medial plantar tubercle; Mt II, articular surface for the second metatarsal; Mt III, articular surface for the third metatarsal; Mt IV + Mt V, articular surface for the fourth and fifth metatarsals; na, articular surface for the navicular; pt, plantar tuberosity; tl, articular surface for the talus. Scale bars equal 0.5 cm</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Metatarsals. a. *Am. leptorhynchus* ITD-VF-12252, ITD-VF-12268, ITD-VF-12235, ITD-VF-12270 and ITD-VF-10822; b. *Ai. fulgens*; c. *V. vulpes*; d. *Me. meles*; a-d. fifth to first left metatarsals in dorsal (above) and medial (below) views. Abbreviations: cu, articular surface for the cuboid; ec, articular surface for the ectocuneiform; en, articular surface for the entocuneiform; gfl, groove for the m. fibularis longus; lt, lateral tubercle; me, articular surface for the mesocuneiform; mt, medial tubercle. Scale bars equal 0.5 cm</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Phalanges. a, e, i. *Am. leptorhynchus* ITD-VF-10204b-c and ITD-VF-10204; b, f. *Ai. fulgens*; e, g. *C. lupus*; d, h, j. *Me. meles*; a-d. proximal phalanx in dorsal, lateral, and plantar views; e-h. middle phalanx in dorsal, lateral, plantar, and proximal views; i-j. distal phalanx in lateral view. Abbreviations: ah, asymmetrical head; fdp, groove for the flexor digitorum profundus. Scale bars equal 0.5 cm</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 9.** Plot of the two first axes of the PCA performed from the morphofunctional indices listed in Tables 4 and 5, showing the distribution of compared carnivoran specimens and *Am. leptorhynchus* (based on the mean value from the different fossil specimens) in the morphospace</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 10.** Plot of the two first axes of the PCA performed from the osteological ratios from the distal tibia, talus and calcaneus combined, from data collected by Marsh et al. (2021). The yellow circle indicates the position in this morphospace of a hypothetical individual of *Am. leptorhynchus* having the average values in the osteological ratios</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 11.** Results of the four leave-one-out cross-validated linear discriminant analyses performed on the osteological ratios from the distal tibia, talus, and calcaneus combined and for each bone separately, from data collected by Marsh et al. (2021)</figcaption></figure>
Tables

<table><caption>**Table 1.** Specimens and ecological categorisations of extant carnivorous analysed in this study</caption><thead><tr><th>Family</th><th>Species</th><th>Specimen</th><th>Origin</th><th>Body size (kg)</th><th>Substrate preference and locomotor mode</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="4">Procyonidae</td><td><i>Bassariscus astutus</i> Lichtenstein, 1830</td><td>MNHN 1854 – 328</td><td>Wild</td><td>0.9–1.1</td><td>Scansorial</td><td>Kays (2009);</td></tr><tr><td><i>Nasua nasua</i> (Linnaeus, 1766)</td><td>MNCN M21728</td><td>Captive</td><td>2–7.2</td><td>Scansorial</td><td>Tarquini et al. (2019)</td></tr><tr><td><i>Potos flavus</i> (Geoffroy Saint-Hilaire, 1824)</td><td>MNHN CG1995-1240</td><td>Wild</td><td>1.4–4.5</td><td>Tree dweller</td><td></td></tr><tr><td><i>Procyon lotor</i> (Linnaeus, 1758)</td><td>MNCN 19506</td><td>Captive</td><td>2.7–10.4</td><td>Terrestrial climber</td><td></td></tr><tr><td rowspan="2">Ailuridae</td><td><i>Ailurus fulgens</i> Cuvier, 1825</td><td>MNCN 14231</td><td>Captive</td><td>3–6</td><td>Tree dweller</td><td>Wei and Zhang (2009); Fabre et al. (2015)</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="4">Mustelidae</td><td><i>Galictis vittata</i> (Schreber, 1776)</td><td>MNHN-ZN-2001-1971</td><td></td><td>1.4–3.3</td><td>Terrestrial generalist</td><td>Larivière and Jennings (2009)</td></tr><tr><td><i>Gulo gulo</i> (Linnaeus, 1758)</td><td>MAV-469</td><td>Wild</td><td>11–18</td><td>Terrestrial climber</td><td></td></tr><tr><td><i>Martes foina</i> (Erxleben, 1777)</td><td>MNCN 3731</td><td>Wild</td><td>1.1–2.3</td><td>Scansorial</td><td></td></tr><tr><td><i>Meles meles</i> (Linnaeus, 1758)</td><td>MNCN 80</td><td>Wild</td><td>10–16</td><td>Terrestrial semi-fossorial</td><td></td></tr><tr><td rowspan="2">Ursidae</td><td><i>Tremarctos ornatus</i> (Cuvier, 1825)</td><td>MAV-1661</td><td>Captive</td><td>60–175</td><td>Scansorial</td><td>Garshelis et al. (2009)</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">Canidae</td><td><i>Canis lupus</i> Linnaeus, 1758</td><td>MNCN 16118</td><td>Wild</td><td>41<sup>b</sup>-62</td><td>Terrestrial cursorial</td><td>Van Valkenburgh (1987);</td></tr><tr><td><i>Vulpes vulpes</i> (Linnaeus, 1758)</td><td>MNCN 17211</td><td>Wild</td><td>3–14</td><td>Terrestrial cursorial</td><td>Van Valkenburgh (1990);</td></tr><tr><td><i>Nyctereutes procyonoides</i> (Gray, 1834)</td><td>MAV-992</td><td>Captive</td><td>2.9–12.4</td><td>Terrestrial generalist</td><td>Sillero- Zubiri (2009)</td></tr><tr><td rowspan="2">Nandinidae</td><td><i>Nandinia binotata</i> (Gray, 1830)</td><td>MNHN 1891 – 1158</td><td>Wild</td><td>1.2–3</td><td>Tree dweller</td><td>Gaubert (2009)</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Viverridae</td><td><i>Genetta genetta</i> (Linnaeus, 1758)</td><td>MNCN 5444</td><td>Captive</td><td>1.4–2.6</td><td>Scansorial</td><td>Jennings and Véron (2009)</td></tr></tbody></table>
<table><caption>**Table 2.** Definitions of substrate preference and locomotor mode categories, adapted from Van Valkenburgh (1987) and Tarquini et al. (2017)</caption><thead><tr><td>Substrate preference and locomotor mode</td><td>Definition</td></tr></thead><tbody><tr><td>Tree dweller</td><td>Spends majority of the time on trees for both food and shelter</td></tr><tr><td>Scansorial</td><td>Spends time both on trees and on the ground without a clear preference for either one</td></tr><tr><td>Terrestrial climber</td><td>Spends majority of the time on the ground, only climbs for refuge or eventual feeding</td></tr><tr><td>Terrestrial generalist</td><td>Moves on the ground but without any specialization, rarely or never climbs or runs fast</td></tr><tr><td>Terrestrial cursorial</td><td>Travels far or fast on the ground</td></tr><tr><td>Terrestrial semi-fossorial</td><td>Spends majority of the time on the ground and digs regularly for both food and shelter</td></tr></tbody></table>
<table><caption>**Table 3.** Abbreviations (Abbr.) and descriptions of measurements taken on the hindlimb</caption><thead><tr><th>Abbr.</th><th>Description</th><th>Abbr.</th><th>Description</th></tr></thead><tbody><tr><td colspan="2"><strong>Coxal bone</strong></td><td colspan="2"><strong>Talus</strong></td></tr><tr><td>ID</td><td>Iliac dorsoventral depth</td><td>TALD</td><td>Talar trochlea depth</td></tr><tr><td>IL</td><td>Ilium length</td><td>TALW</td><td>Talar condyles width (TALW1+TALW2)</td></tr><tr><td>PEL</td><td>Total length of coxal bone</td><td>TALW1</td><td>Medial talar condyle width</td></tr><tr><td colspan="2"><strong>Femur</strong></td><td>TALW2</td><td>Lateral talar condyle width</td></tr><tr><td>FL</td><td>Femur length</td><td>TALH1</td><td>Dorsoplantar length of the talar head</td></tr><tr><td>FGT</td><td>Length of distal extension of the greater trochanter of the femur</td><td>TALH2</td><td>Mediolateral length of the talar head</td></tr><tr><td>FHM</td><td>Femoral head mediolateral width</td><td>TALC</td><td>Lateral talar condyle curve</td></tr><tr><td>FHC</td><td>Femoral head craniocaudal width</td><td>TALL</td><td>Lateral talar condyle length</td></tr><tr><td>FW</td><td>Femur midshaft mediolateral width</td><td colspan="2"><strong>Calcaneus</strong></td></tr><tr><td>FEMW</td><td>Femur distal trochlea mediolateral width</td><td>CL</td><td>Calcaneus proximodistal length</td></tr><tr><td>FEMD</td><td>Femur distal trochlea craniocaudal depth</td><td>CTW</td><td>Calcaneal tuber mediolateral width at the midpoint of its length</td></tr><tr><td>FD</td><td>Femur distal epiphysis craniocaudal depth</td><td>CTH</td><td>Calcaneal tuber proximo-distal length</td></tr><tr><td>FEB</td><td>Epicondylar breadth of the distal femur</td><td>CW</td><td>Calcaneus width at the level of the sustentaculum and peroneal tubercle</td></tr><tr><td colspan="2"><strong>Tibia</strong></td><td>CEF</td><td>Calcaneal ectal facet proximodistal length</td></tr><tr><td>TL</td><td>Tibia length</td><td colspan="2"><strong>Cuboid</strong></td></tr><tr><td>TTL</td><td>Length of distal extension of the tibial tuberosity</td><td>CBL</td><td>Cuboid length</td></tr><tr><td>THC</td><td>Tibial head craniocaudal width</td><td colspan="2"><strong>Third metatarsal</strong></td></tr><tr><td>THM</td><td>Tibial head mediolateral width</td><td>MT3L</td><td>Third metatarsal length</td></tr><tr><td>TCW</td><td>Tibial midshaft craniocaudal width</td><td>MT3W</td><td>Third metatarsal width</td></tr><tr><td>TMW</td><td>Tibial midshaft mediolateral width</td><td colspan="2"><strong>Proximal phalanx</strong></td></tr><tr><td></td><td></td><td>PPL</td><td>Third proximal phalanx length</td></tr></tbody></table>
<table><caption>**Table 4.** Abbreviations (Abbr.) and description of the morpho-functional indices</caption><thead><tr><td>Abbr.</td><td>Index</td><td>Calculation</td><td>Reference(s)</td></tr></thead><tbody><tr><td>IRD</td><td>Iliac relative depth</td><td>ID / IL</td><td>a</td></tr><tr><td>GLU</td><td>Gluteal index</td><td>FGT / FL</td><td>d</td></tr><tr><td>FHI</td><td>Femoral head index</td><td>FHM / FHC</td><td>e</td></tr><tr><td>FCP</td><td>Femoral condyles caudal projection</td><td>FD / FL</td><td></td></tr><tr><td>FTD</td><td>Relative depth of the femoral trochlea</td><td>FEMD / FEMW</td><td>c</td></tr><tr><td>TTI</td><td>Tibial tuberosity index</td><td>TTL / TL</td><td>d</td></tr><tr><td>THI</td><td>Tibial head index</td><td>THC / THM</td><td>a, e</td></tr><tr><td>TARD</td><td>Relative depth of the talar condyle</td><td>TALD / TALW</td><td>b, c</td></tr><tr><td>TACS</td><td>Talar condyle symmetry</td><td>TALW1 / TALW</td><td>c</td></tr><tr><td>TACV</td><td>Talar condyle curvature</td><td>TALC / TALL</td><td>c</td></tr><tr><td>CTC</td><td>Tuber calcanei – calcaneus length ratio</td><td>CTH / CL</td><td></td></tr><tr><td>CRW</td><td>Calcaneus relative width</td><td>CW / CL</td><td></td></tr><tr><td>CEFL</td><td>Calcaneal ectal facet relative length</td><td>CEF / CL</td><td></td></tr><tr><td>MT3R</td><td>Third metatarsal robustness index</td><td>MTC3L / MT3W</td><td>c</td></tr><tr><td>MT3L</td><td>Relative length of the third metatarsal</td><td>MT3L / (FL + TL + MT3L)</td><td>a, b, c, d, e</td></tr><tr><td>CI</td><td>Crural index</td><td>TL / FL</td><td>a, e</td></tr><tr><td>CMT</td><td>Calcaneus–metatarsal ratio</td><td>CL / MT3L</td><td>c</td></tr><tr><td>MTP</td><td>Proximal phalanx–third metatarsal ratio</td><td>PPL / MT3L</td><td></td></tr></tbody></table>

References: <sup>a</sup>Taylor (1976); <sup>b</sup>Van Valkenburgh (1987); <sup>c</sup>Carrano (1997); <sup>d</sup>Samuels et al. (2013); <sup>e</sup>Ercoli and Youlatos (2016)
<table><caption>**Table 5.** Hindlimb morpho-functional indices (mean value for the specimens of *Am. leptorhynchus*)</caption><thead><tr><th>Species</th><th>IRD</th><th>GLU</th><th>FHI</th><th>FCP</th><th>FTD</th><th>TTI</th><th>THI</th><th>TARD</th><th>TACS</th><th>TACV</th><th>CTC</th><th>CRW</th><th>CEFL</th><th>MT3R</th><th>MT3L</th><th>CI</th><th>CMT</th><th>MTP</th></tr></thead><tbody><tr><td>Ai. fulgens</td><td>0.47</td><td>0.20</td><td>1.13</td><td>0.14</td><td>0.12</td><td>0.31</td><td>0.81</td><td>0.11</td><td>0.33</td><td>1.44</td><td>0.65</td><td>0.51</td><td>0.32</td><td>0.09</td><td>0.14</td><td>0.93</td><td>0.75</td><td>0.56</td></tr><tr><td><strong>Am. leptorhynchus</strong></td><td><strong>0.36</strong></td><td><strong>0.09</strong></td><td><strong>1.00</strong></td><td><strong>0.16</strong></td><td><strong>0.15</strong></td><td><strong>0.37</strong></td><td><strong>0.86</strong></td><td><strong>0.23</strong></td><td><strong>0.49</strong></td><td><strong>1.51</strong></td><td><strong>0.68</strong></td><td><strong>0.56</strong></td><td><strong>0.37</strong></td><td><strong>0.11</strong></td><td><strong>0.15</strong></td><td><strong>0.94</strong></td><td><strong>0.62</strong></td><td><strong>0.44</strong></td></tr><tr><td>B. astutus</td><td>0.34</td><td>0.12</td><td>0.93</td><td>0.15</td><td>0.15</td><td>0.34</td><td>0.82</td><td>0.22</td><td>0.60</td><td>1.40</td><td>0.61</td><td></td><td></td><td></td><td>0.13</td><td>0.15</td><td>1.02</td><td>0.65</td><td>0.49</td></tr><tr><td>C. lupus</td><td>0.63</td><td>0.20</td><td>0.71</td><td>0.21</td><td>0.46</td><td>0.27</td><td>1.10</td><td>0.24</td><td>0.43</td><td>1.53</td><td>0.70</td><td>0.39</td><td>0.22</td><td>0.10</td><td>0.17</td><td>1.01</td><td>0.62</td><td>0.38</td></tr><tr><td>Ga. vittata</td><td>0.35</td><td>0.11</td><td>0.92</td><td>0.17</td><td>0.20</td><td>0.37</td><td>0.86</td><td>0.12</td><td>0.53</td><td>1.45</td><td>0.65</td><td>0.57</td><td>0.34</td><td>0.11</td><td>0.15</td><td>0.93</td><td>0.72</td><td>0.46</td></tr><tr><td>Ge. genetta</td><td>0.40</td><td>0.09</td><td>0.70</td><td>0.16</td><td>0.15</td><td>0.34</td><td>0.92</td><td>0.19</td><td>0.41</td><td>1.49</td><td>0.59</td><td>0.48</td><td>0.22</td><td>0.08</td><td>0.17</td><td>1.06</td><td>0.61</td><td>0.27</td></tr><tr><td>Gu. gulo</td><td>0.59</td><td>0.20</td><td>0.74</td><td>0.19</td><td>0.08</td><td>0.36</td><td>1.01</td><td>0.19</td><td>0.39</td><td>1.57</td><td>0.70</td><td>0.53</td><td>0.36</td><td>0.13</td><td>0.16</td><td>0.97</td><td>0.80</td><td></td></tr><tr><td>Ma. foina</td><td>0.43</td><td>0.19</td><td>0.86</td><td>0.14</td><td>0.13</td><td>0.32</td><td>0.82</td><td>0.11</td><td>0.36</td><td>1.45</td><td>0.70</td><td>0.49</td><td>0.33</td><td>0.08</td><td>0.18</td><td>1.03</td><td>0.54</td><td>0.40</td></tr><tr><td>Me. meles</td><td>0.50</td><td>0.18</td><td>0.76</td><td>0.17</td><td>0.26</td><td>0.49</td><td>0.89</td><td>0.22</td><td>0.65</td><td>1.37</td><td>0.71</td><td>0.51</td><td>0.34</td><td>0.14</td><td>0.13</td><td>0.79</td><td>0.94</td><td>0.41</td></tr><tr><td>N. binotata</td><td>0.34</td><td>0.12</td><td>0.92</td><td>0.15</td><td>0.22</td><td>0.36</td><td>0.86</td><td>0.18</td><td>0.52</td><td>1.44</td><td>0.63</td><td>0.54</td><td>0.37</td><td></td><td>0.14</td><td>0.97</td><td>0.79</td><td>0.53</td></tr><tr><td>Na. nasua</td><td>0.63</td><td>0.24</td><td>0.99</td><td>0.17</td><td>0.24</td><td>0.38</td><td>0.94</td><td>0.27</td><td>0.46</td><td>1.44</td><td>0.68</td><td>0.56</td><td>0.34</td><td>0.12</td><td>0.13</td><td>0.95</td><td>0.88</td><td>0.44</td></tr><tr><td>Ny. procyonoides</td><td>0.47</td><td>0.15</td><td>0.81</td><td>0.21</td><td>0.43</td><td>0.29</td><td>0.92</td><td>0.40</td><td>0.45</td><td>1.50</td><td>0.65</td><td>0.47</td><td>0.28</td><td>0.09</td><td>0.18</td><td>1.07</td><td>0.56</td><td>0.35</td></tr><tr><td>Po. flavus</td><td>0.32</td><td>0.10</td><td>0.93</td><td>0.14</td><td>0.15</td><td>0.36</td><td>0.80</td><td>0.14</td><td>0.30</td><td>1.71</td><td>0.63</td><td>0.50</td><td>0.35</td><td>0.11</td><td>0.14</td><td>1.01</td><td>0.81</td><td>0.57</td></tr><tr><td>T. ornatus</td><td>0.67</td><td>0.21</td><td>0.74</td><td>0.16</td><td>0.18</td><td>0.31</td><td>0.91</td><td>0.09</td><td>0.51</td><td>1.08</td><td>0.74</td><td>0.66</td><td>0.38</td><td>0.14</td><td>0.10</td><td>0.79</td><td>1.01</td><td>0.60</td></tr><tr><td>V. vulpes</td><td>0.57</td><td>0.16</td><td>0.83</td><td>0.19</td><td>0.37</td><td>0.22</td><td>1.06</td><td>0.42</td><td>0.40</td><td>1.46</td><td>0.65</td><td>0.43</td><td>0.26</td><td>0.09</td><td>0.19</td><td>1.07</td><td>0.55</td><td>0.36</td></tr></tbody></table>
<table><caption>Table 5. Classification results from leave-one-out cross-validated linear discriminant analysis (LDA) of osteological variables from the distal tibia, talus and calcaneus, among hindfoot reversal categories and predictions for *Am. leptorhynchus* specimens</caption><thead><tr><th></th><th>Accuracy</th><th>Kappa index</th><th>Non</th><th>Partial</th><th>Reverser</th><th><i>Am. leptorhynchus</i></th></tr></thead><tbody><tr><td rowspan="4">Prediction for the tibia according to the LDA</td><td rowspan="4">0.65</td><td rowspan="4">0.38</td><td>Non</td><td>0</td><td>2</td><td>0</td><td>0</td></tr><tr><td>Partial</td><td>5</td><td>26</td><td>12</td><td>0</td></tr><tr><td>Reverser</td><td>1</td><td>7</td><td>26</td><td>13</td></tr><tr><td><b>Total</b></td><td><b>6</b></td><td><b>35</b></td><td><b>38</b></td><td><b>13</b></td></tr><tr><td rowspan="4">Prediction for the talus according to the LDA</td><td rowspan="4">0.79</td><td rowspan="4">0.63</td><td>Non</td><td>4</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Partial</td><td>0</td><td>24</td><td>6</td><td>8</td></tr><tr><td>Reverser</td><td>2</td><td>7</td><td>30</td><td>2</td></tr><tr><td><b>Total</b></td><td><b>6</b></td><td><b>31</b></td><td><b>36</b></td><td><b>10</b></td></tr><tr><td rowspan="4">Prediction for the calcaneus according to the LDA</td><td rowspan="4">0.69</td><td rowspan="4">0.46</td><td>Non</td><td>6</td><td>1</td><td>0</td><td>0</td></tr><tr><td>Partial</td><td>0</td><td>19</td><td>10</td><td>0</td></tr><tr><td>Reverser</td><td>0</td><td>12</td><td>26</td><td>8</td></tr><tr><td><b>Total</b></td><td><b>6</b></td><td><b>32</b></td><td><b>36</b></td><td><b>8</b></td></tr><tr><td rowspan="4">Prediction for the three bones according to the LDA</td><td rowspan="4">0.77</td><td rowspan="4">0.60</td><td>Non</td><td>5</td><td>0</td><td>1</td><td>0</td></tr><tr><td>Partial</td><td>0</td><td>20</td><td>7</td><td>0</td></tr><tr><td>Reverser</td><td>1</td><td>6</td><td>25</td><td>1</td></tr><tr><td><b>Total</b></td><td><b>6</b></td><td><b>26</b></td><td><b>33</b></td><td><b>1</b></td></tr></tbody></table>