# Revisiting the aerobic capacity of Notosuchia
(Crocodyliformes, Mesoeucrocodylia)

Mariana V A Sena, Felipe C Montefeltro, Thiago S Marinho, Max C Langer, Thiago S Fachini, Andre E P Pinheiro, Alessandra S Machado, Ricardo T Lopes, Romain Pellarin, Juliana M Sayao, et al.

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Mariana V A Sena, Felipe C Montefeltro, Thiago S Marinho, Max C Langer, Thiago S Fachini, et al. Revisiting the aerobic capacity of Notosuchia (Crocodyliformes, Mesoeucrocodylia). Lethaia, 2024, 57 (4), pp.1-8. 10.18261/let.57.4.6. hal-04964405

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# Revisiting the aerobic capacity of Notosuchia
(Crocodyliformes, Mesoeucrocodylia)

MARIANA V. A. SENA, FELIPE C. MONTEFELTRO, THIAGO S. MARINHO, MAX C. LANGER, THIAGO S. FACHINI, ANDRE E. P. PINHEIRO, ALESSANDRA S. MACHADO, RICARDO T. LOPES, ROMAIN PELLARIN, JULIANA M. SAYAO, GUSTAVO R. OLIVEIRA AND JORGE CUBO

LETHAIA

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This study examines the metabolic rates of notosuchian crocodyliforms. Although predominantly composed of terrestrial forms, the group also encompasses the putatively semi-aquatic itasuchids. The notosuchian record is particularly rich in Brazil and Argentina, which provides a basis for understanding their ecology and physiology. This research expands a previous study that investigated the aerobic capacities of these organisms, considering blood flow rate (Q) in the femoral nutrient artery as a proxy to infer their maximal aerobic metabolic rates, critical for understanding peak aerobic performance during strenuous activities. Femoral nutrient foramina act as conduits for the major blood supply to the bone and are correlated with aerobic capacity, providing insights into the physiological capabilities of the studied taxa. This study revisits and expands upon a previous dataset by including additional taxa and provides more precisely adjusted mass-independent maximal metabolic rate predictions for the sample of Notosuchia. We incorporate data from a broader spectrum of extant tetrapods and employ Phylogenetic Eigenvector Maps (PEM) to refine our analyses. PEM allow the inclusion of an auxiliary predictor related to the response variable, alongside phylogenetic eigenvectors. The results indicate notosuchians possessed higher mass-independent maximal metabolic rates (MMRs) than extant Crocodylia, and lower than those of extant varanid lizards, which suggests an elevated aerobic capacity, enabling a more active lifestyle compared to extant crocodylians. The study addresses previous miscalculations for the body mass exponent on the MMR unit. We introduce Q values based on the nutrient femoral artery lumen area instead of the total area of the nutrient foramen, and provide a corrected MMR for *Crocodylus porosus*, allowing more accurate predictions of notosuchian aerobic capacities. This research reveals metabolic flexibility within crocodyliforms, with an elevated aerobic capacity allowing notosuchians to sustain vigorous activities, possibly related to their terrestrial adaptations and active foraging strategies. ☐ Bone, femoral nutrient foramina, metabolic rates, Notosuchia, Phylogenetic Eigenvector Maps

Mariana Sena [mari.araujo.sena@gmail.com] and Jorge Cubo [jorge.cubo_garcia@sorbonne-universite.fr], Sorbonne Université, Muséum national d'Histoire naturelle, CNRS, Centre de recherche en paléontologie – Paris (CR2P, UMR 7207), 4 Place Jussieu, Paris, BC 104, 75005, France MVAS; Felipe Montefeltro, felipecmontefeltro@gmail.com, Faculdade de Engenharia, Ilha Solteira, Universidade Estadual Paulista (UNESP), School of Engineering, São Paulo State University (UNESP), Rua Monção, 226, Ilha Solteira, São Paulo, 15385-000, Brazil; Thiago Marinho [thiago.marinho@uftm.edu.br] Instituto de Ciências Exatas, Naturais e Educação, Universidade Federal do Triângulo Mineiro, Av. Randolfo Borges Jr. 1400, Uberaba, Minas Gerais, 38064-200, Brazil; Max Langer, [mclanger@ffclrp.usp.br] Laboratório de Paleontologia de Ribeirão Preto, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Av. Bandeirantes, 3900, Ribeirão Preto, São Paulo, 14040-901, Brazil; Thiago Fachini [thiagosfachini84@gmail.com], Laboratório de Paleontologia de Ribeirão Preto, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Av. Bandeirantes, 3900, Ribeirão Preto, São Paulo, 14040-901, Brazil; Andre Pinheiro [andre.eduardo.pinheiro@uerj.br], Laboratório de Paleontologia de São Gonçalo, Faculdade de Formação de Professores, Universidade do Estado do Rio de Janeiro (LAPASGO/FFP/UERJ), Campus São Gonçalo, Rua Dr. Francisco Portela, Bairro do Patronato, São Gonçalo, Rio de Janeiro 24435-005, Brazil; Alessandra Machado [ales-sandramachado192@gmail.com] and Ricardo Lopes [rlopes@coppe.ufrj.br], Laboratório de Instrumentação Nuclear – COPPE/UFRJ, Universidade Federal do Rio de Janeiro, Ilha do Fundão, Rio de Janeiro, 21941-596, Brazil; Juliana Sayao, [jmsayao@mn.ufrj.br] Museu Nacional do Rio de Janeiro, Universidade Federal do Rio de Janeiro, Quinta da Boa Vista s/nº, São Cristóvão, Rio de Janeiro, Rio de Janeiro, 20940-040, Brazil; Gustavo Oliveira [gustavo.roliveira@ufrpe.br], Laboratório de Paleontologia e Sistemática, Departamento de Biologia, Universidade Federal Rural de Pernambuco, Rua Dom Manoel de Medeiros, s/nº, Dois Irmãos, Recife, Pernambuco, 52171-900, Brazil; manuscript received on 28/04/2024; manuscript accepted on 27/09/2024 manuscript published on 31/01/2025 in Lethaia 57(4).
Notosuchian crocodyliforms are known for their high diversity during the Late Cretaceous, exhibiting a broad range of anatomical modifications including specialized maxillo-mandibular apparatuses with dentitions that resemble those of mammals (O'Connor *et al.* 2010; Ōsi, 2013; Melstrom & Irmis 2019). Predominantly terrestrial, forms like the *Araripesuchus* and *Baurusuchus* species show adaptations to such habitats, as laterally compressed snouts and anteriorly-placed nostrils, and limb modifications indicative of cursorial locomotion, including closer radioulnar spacing and weakly sigmoidal femora with less torsion of the femoral head (Andrade & Bertini 2008; Nascimento & Zaher 2012; Godoy *et al.* 2016; Fernández Dumont *et al.* 2020). Conversely, forms like *Pepesuchus* were likely semi-aquatic (Pinheiro *et al.* 2018; Sena *et al.* 2018; Ruiz *et al.* 2024). Especially diverse in the Cretaceous of Gondwana, notosuchians are well represented in the Bauru Basin, southeastern Brazil, with over thirty described species (Ruiz *et al.* 2021; Langer *et al.* 2022; Aubier *et al.* 2023; Martins *et al.* 2024; Borsoni *et al.* 2024).

Recently published studies on crocodyliform
metabolism conclude that marine lifestyles impose
strong selective pressures favoring larger body sizes
in crocodyliforms, compared to those living on land
or in semi-aquatic forms (Gearty & Payne 2020). This
was attributed to physiological constraints imposed by
the marine environment, such as the need for greater
diving capacity and efficient heat retention. Cubo
et al. (2020, 2023) inferred notosuchian metabolic
rates using cell and vascular densities, and the proba-
bility of endothermy using vascular canal size, in long
bone thin sections. This approach allowed to infer an
ectothermic thermometabolism and moderate rest-
ing metabolic rates, akin to those measured in living
varanid lizards. Sena et al. (2023) investigated the
notosuchian aerobic capacities, using femoral nutri-
ent foramina area as a measure of the maximum rate
of oxygen consumption (VO2max). This rate is cru-
cial to understand an animal's capacity for sustained
aerobic, high-intensity activities. Previous works by
Seymour et al. (2012) Seymour (2013) showed that
VO2 max is a key indicator of tetrapod's peak aerobic
performances. Later studies confirmed the potential
links between nutrient foramina size and physiolog-
ical traits (e.g. Allan et al. 2014; Seymour et al. 2024).
The concept of maximum metabolic rate is central to
this discussion. It refers to the animal's capacity to sus-
tain vigorous activities, with more athletic taxa gener-
ally maintaining higher rates (Hayes et al. 2018; Wang
et al. 2001; Weibel et al. 2004). It is hypothesized that
variation in blood perfusion reflects differences in the
metabolic capacity of species, with animals exhibiting

The concept of maximum metabolic rate is central to
this discussion. It refers to the animal's capacity to sus-
tain vigorous activities, with more athletic taxa gener-
ally maintaining higher rates (Hayes et al. 2018; Wang
et al. 2001; Weibel et al. 2004). It is hypothesized that
variation in blood perfusion reflects differences in the
metabolic capacity of species, with animals exhibiting

higher levels of locomotory activity expected to have
higher blood perfusion rates through larger blood
vessels (Weibel et al. 2004). This relationship can be
assessed using the nutrient foramina opening area in
long bones, which suggests that more athletic animals
had higher perfusion rates required for bone remod-
eling to repair microfractures in response to stress
during locomotion (Seymour et al. 2012). Hence, the
metabolism of extinct species has been investigated by
measuring foramina periosteal opening on the shafts
of their long bones and comparing them with data on
extant data (e.g. Cubo et al. 2024; Knaus et al. 2021;
Seymour et al. 2024; Varela et al. 2024).Ventilatory
and cardiovascular processes associated with oxygen
transport and delivery are augmented during activity.
Oxygen transport during the peak metabolic activity
involves several steps through convection of oxygen
into the lungs by ventilation; oxygen diffusion from
the lung air into the bloodstream, convection of
oxygen-rich blood to peripheral tissues, and diffusion
of oxygen to the mitochondria within cells where it
is used for energy production (Bennett & John-Alder
1984; Farmer & Hicks 2000; Weibel et al. 2004).

Sena et al. (2023) concluded that notosuchians
might have been more athletic than extant ambush
crocodylians, possibly due to their parasagittal pos-
ture (as inferred by their postcranial anatomy) ena-
bling more active behaviours. Building on these
findings, our current study expands the analysis of
maximal metabolic rates in notosuchians by incorpo-
rating nine new tetrapod taxa to the previous database
(Sena et al. 2023) and updating the VO2max assess-
ment for *Crocodylus porosus*. This aims to complement
our knowledge about the physiological constraints of
these crocodyliforms.

**Material and methods**

Dataset scope

In this investigation, we reassess previous inferences
of mass-independent MMR retrodictions for seven
notosuchian taxa. Our updated dataset includes 29
extant tetrapod species, including 20 species from
Seymour *et al.* (2012) and nine newly added to that of
Sena *et al.* (2023). This includes: two extant crocody-
lian species, *Alligator mississippiensis* and *Crocodylus
niloticus*, three mammals (*Papio hamadryas*, *Helogale
parvula*, and *Acinonyx jubatus*) and four lizards
(*Tupinambis teguixin*, *Iguana iguana*, *Amphibolurus
barbatus*, and *Varanus salvator*) (Supplementary
Information). The additional taxa were selected based
on specimens from the palaeontological collection
of the National Museum of Natural History in Paris (MNHN).

## Phylogenetic framework

We derived phylogenetic relationships from the literature, employing Zurano et al. (2019) for Cetartiodactyla, Upham et al. (2019) for other mammals, Pyron et al. (2013) and Vidal & Hedges (2005) for squamates in general, Villa et al. (2018) for varanid lizards, and Pinheiro et al. (2021) for Mesoeucrocodylia. Branch length data for extant taxa were gathered from the Time Tree of Life (timetree.org, accessed 31 January, 2024), and those for the extinct ones from the Paleobiology Database (paleobiodb.org, accessed 31 January, 2024) and Buffrénil et al. (2021).

We adopted the Notosuchia clade definition of Leardi et al. (2024), encompassing Eunotosuchia and Sebecia, with their compositions as per established systematics (e.g. Larsson & Sues 2007; Pinheiro et al. 2018).

## Morphological and physiological measurements and calculations

We focused on the area of the femoral nutrient foramina, which are responsible for more than half of the total blood flow to the bone, as an indicator of aerobic capacity, following the methodology proposed by Seymour et al. (2012, 2019). To calculate the blood flow rate (Q), we determined the cross-sectional areas at the periosteal openings of the foramina by segmenting the nutrient foramina from tomographic scans of the femora of our sampled notosuchians using Mimics software at the MNHN. For specimens with multiple foramina, we added the corresponding areas and computed the radius of the resulting area assuming circularity.

We estimated the nutrient artery lumen area, assuming that it corresponds to 20% of the nutrient foramen area. In this, we followed Hu et al. (2021), who found that the lumen of a pressurized nutrient artery occupies 20% of the foramen area. In our previous work, we used the total area of the nutrient foramen, leading to an overestimation of our measurements. The femoral nutrient arterial radii were determined by the circle area equation, considering the nutrient arterial lumen areas. The blood flow rate (Q; mL/s), based on the arterial internal radius (r; cm) of the species in our dataset was calculated using the equation proposed by Seymour et al. (2019):

$$ \log Q = -0.20 \log r_i^2 + 1.91 \log r_i + 1.82 $$

The femoral nutrient foramina areas for notosuchians were sourced from Sena et al. (2023), and previously published values for extant species are provided in the Supplementary Information. The inferred body masses (BM) of the sampled notosuchians were published by Woodward et al. (2024) and were obtained applying the equation:

$$ BM = \exp(3.6427*\ln(\text{femur length}) - 14.66296) $$

## Phylogenetic comparative methods

A palaeobiological inference model for maximum metabolic rate (MMR) was developed using phylogenetic eigenvector maps (PEMs; see Guénard et al. 2013). The PEMs were constructed using the ‘MPSEM’ package (Guénard et al. 2013) in R (R Core Team 2023). This model enabled us to estimate the MMR of notosuchians, along with corresponding 95% confidence intervals. The normality of the residuals was checked using the Shapiro-Wilk test.

## Metabolic Rate Estimations Unit

Our current methodology diverges from that of Sena et al. (2023), in which a weighted average of exponents was employed specifically for notosuchians. Although dividing the MMR by body mass yields a mass-specific MMR, this parameter remains influenced by body mass. A mass-independent MMR is obtained by dividing the MMR by body mass raised to an exponent determined through a regression analysis of known MMR values and body masses (White & Seymour 2005; Seymour 2013). The method applied by Sena et al. (2023) calculates mass-independent maximum metabolic rates using different exponents, 0.829 for sauropsids (Bennett & Dawson 1976; Seymour et al. 2013) and 0.87 for synapsids (White & Seymour 2005) to consider the physiological and metabolic differences between these groups. However, this approach involved using different units for different data points in a single statistical analysis and this is mathematically questionable.

Here, we employed a 0.84 exponent to body mass unit (expressed in grams) for all data points. This exponent corresponds to the phylogenetic mean computed recovered in using Mesquite (Maddison & Maddison 2014). This generates a single character matrix from distinct extant species exponents (0.829 for sauropsids 0.87 for synapsids) and a question mark for extinct taxa. Then we used the functions for extinct taxa, with the exponent recovered by using the ‘Reconstruct ancestral states’, function inside ‘Trace’ tool and ‘character history source’ to obtain
the phylogenetic mean of the exponent of the sample. This approach is more aligned with evolutionary principles and apparently more closely reflects the characteristics of the studied crocodyliforms. In this case, the exponent is 0.84, so mass-independent MMR data have units of mL O₂ h⁻¹g⁻⁰.⁸⁴.

## Results

For this study, the mass-independent MMR for *Crocodylus porosus* was corrected for 1.11 mL O₂ h⁻¹g⁻⁰.⁸⁴ (Supplementary Information) and new data for two crocodylians, *Alligator mississippiensis* and *Crocodylus niloticus*, were included in our dataset. The retrodictions show a similar mass-independent values for all studied notosuchians with a mean value of approximately 1.55 mL O₂ h⁻¹g⁻⁰.⁸⁴ using phylogeny+Q inference model (Table 1). Notosuchia mass-independent MMRs are higher than those measured for extant crocodylians, and lower than those of varanid lizards. The MMR lower confidence interval of notosuchians includes the sampled crocodylians and other sauropsids, such as *Iguana iguana*, the blue-tongued skink, *Tiliqua scincoides*, and the Eastern bearded dragon *Amphibolurus barbatus*.

The phylogeny+Q inference model has an adjusted R² of 0.97 and AICc of 34.43 (*p* < 0.001). We performed a leave-one-out cross-validation test to check the robustness of the model. Then, we used the

non-parametric Wilcoxon signed-rank test to compare the MMR values measured in extant species and compiled by Seymour *et al.* (2012) (red, purple and orange squares in Fig. 1). We did not find significant differences between the observed and the predicted values (*p*-value = 0.3579). Thus the retrodictions for Notosuchia made by our model are robust.

## Discussion

In this study, we found that the mass-independent Metabolic Rate (MMR) values (1.55 mL O₂ h⁻¹g⁻⁰.⁸⁴) are higher than those (0.7 mL O₂ h⁻¹g⁻⁰.⁸⁵) previously reported by Sena *et al.* (2023). Our analysis, based on Phylogenetic Eigenvector Maps (PEM), suggests that the phylogeny+Q model is effective for predicting mass-independent MMR. These more precise retrodictions reveal that Notosuchia likely possessed a more elevated aerobic capacity compared to what was previously retrodicted by Sena *et al.* (2023).

This study corrects the mass-independent MMR of *Crocodylus porosus* using a value (0.137 mL O₂ h⁻¹g⁻¹) higher than the previous miscalculations (0.021 mL O₂ h⁻¹g⁻¹). Recovering the phylogenetic mean of 0.84 from our dataset as the exponent for body mass simplifies our analysis and offers a broader overview of metabolic scaling from an evolutionary perspective. This approach, however, may not adequately account for the inherent physiological and metabolic adaptations distinguishing endotherms from ectotherms within our sample (Seymour 2013). Such disparities can influence metabolic rates, suggesting that a singular exponent may fall short of capturing the complexity in these variations.

Among the sauropsids, the mass independent MMR range for notosuchians encompass those measured for blue-tongued skinks, *Tiliqua* spp. In scincid and agamid lizards, such as *Tiliqua* and *Amphibolurus*, the unicameral lungs are relatively large and highly expandable (Daniels *et al.* 1994). The absence of a muscular diaphragm allows these lungs to expand freely into a substantial portion of the body cavity. Additionally, the shape and volume of the lungs exhibit a certain degree of plasticity due to the abdominal organs, which can compress the caudal and ventral regions of the lungs (Munns & Daniels 2007). This anatomical flexibility may contribute to the unexpectedly relative high MMR values observed in the sampled blue-tongue skinks and agamid lizards.

As ectothermic tetrapods, notosuchians' physiological functions, including resting and maximal oxygen consumption rates, significantly rely on ambient temperatures (Bennett & John-Alder 1984; Cubo *et al.*

<table><caption>**Table 1.** Mass-independent maximum metabolic rate (MMR) of sampled notosuchian taxa retrodicted through phylogenetic eigenvector maps using Q as co-predictor.</caption><thead><tr><th>Species</th><th>Collection number</th><th>Mass-independent MMR (ml O<sup>2</sup> h<sup>-1</sup>g<sup>-0.84</sup>)</th></tr></thead><tbody><tr><td>Campinasuchus_<br>dinizi</td><td>CPPLIP 1847</td><td>1.5809</td></tr><tr><td>Araripesuchus_sp</td><td>LAPEISA-Pal-0032</td><td>1.5737</td></tr><tr><td>Sahitisuchus_<br>fluminensis</td><td>MCT 1835-R</td><td>1.5441</td></tr><tr><td>Sahitisuchus_<br>fluminensis</td><td>MCT 1831-R</td><td>1.5834</td></tr><tr><td>Itasuchus_jesuinoi</td><td>DGM 434-R</td><td>1.5156</td></tr><tr><td>Pissarrachampsa_<br>sera</td><td>LPRP USP 0019</td><td>1.5134</td></tr><tr><td>Pissarrachampsa_<br>sera</td><td>LPRP USP 0019b</td><td>1.4869</td></tr><tr><td>Coronelsuchus_<br>civali</td><td>FFP PG 14</td><td>1.5869</td></tr><tr><td>Uberabasuchus_<br>terrificus</td><td>CPPLIP 501</td><td>1.5631</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 1. Phylogenetic relationships among extant taxa used to construct the mass-independent maximum metabolic rates inference model and the extinct Notosuchia for which we performed retrodictions using *Q̇* as co-predictor. Branches are proportional to time. The tetrapod silhouettes have been modified and adapted from copyright-free images provided by Freepik (http://www.freepik.com).</figcaption></figure>

2020, 2023). This thermal dependence likely shaped their daily activity patterns, enabling periods of intense locomotion and exploration, similar to patterns observed in other ectothermic species such as varanid lizards. Such behavioural flexibility would have been crucial for large notosuchians, such as baurusuchids (e.g. *Pissarrachampsa sera*), to confront predators or actively pursuing prey, as suggested for the group (Montefeltro *et al.* 2020), whereas smaller species (e.g. *Araripesuchus*) might have relied on agility to evade threats.

A burrowing habit has been suggested for a few notosuchians. Vasconcellos & Carvalho (2006) suggest that the holotype of the peirosaurid *Uberabasuchus terrificus* might have been preserved articulated within a burrow. Martinelli *et al.* (2019) described the first Late Cretaceous burrow attributed to a notosuchian, found in sediments of the Serra da Galga Formation in Uberaba, Minas Gerais state, southeastern Brazil. Considering this habit, notosuchians could

remain active throughout the day to reenter their burrows during the late afternoon, with the body temperature falling throughout the evening and night. This behaviour is described for tegu lizards by Milsom *et al.* (2008). Difficulties still arise from the unknown disposition of maintenance processes during activity in determining how long these crocodyliforms could maintain strenuous exercise. This consideration should include anaerobic glycolysis with progressive acidosis and lactate accumulation (Bennett 1978). For now, we can say they have greater endurance than extant crocodylians but likely less than teiid lizards. Nevertheless, the studied notosuchians still display mass-independent MMRs almost half of those values measured in varanid lizards. Carrier (1987) showed that the presence of large vertebral processes supporting the locomotor muscles and unloading of the respiratory systems and the presence of parasagittal members indicate the possession of diaphragmatic muscles and the lateral stability of the vertebral
column, allowing and efficient breathing that is associated with endothermy.

Several notosuchian taxa, including the *Sebecus icaeorhinus*, *Notosuchus terrestris*, *Mariliasuchus amarali*, and *Pakasuchus kapilimai* bear postcraniial modifications that suggest an upright stance and possible cursorial abilities. These features hint at a more efficient terrestrial movement strategy and are reminiscent of those found in certain land-dwelling mammals (Pol 2005; Nascimento & Zaher 2010; Pol *et al.*, 2012; Cotts *et al.* 2016; Godoy *et al.* 2016; Montefeltro 2019). Additionally, the evolution of a mammal-like heterodont dentition occurred independently across several groups, including sphagesaurids, *Araripesuchus* spp., *Malawisuchus mwakasyungutiensis*, and *Chimaerasuchus paradoxus*. This suggests dietary diversity within the groups (Ösi 2013; Pol *et al.* 2014; Melstrom & Irmis 2019).

Whereas baurusuchids, itasuchids, and sebecids are characterized by their large size, other notosuchians are smaller, comparable to medium-sized mammals. This similarity in size implies that these notosuchians might have occupied similar ecological roles in the Gondwanan landmasses as mammals did in the northern continents, indicating a convergent evolutionary path towards filling ecological niches across different geographical regions (O'Connor *et al.* 2010).

Varanid lizards and crocodylians exhibit significant differences in their cardiopulmonary systems, which underlie their distinct aerobic capabilities and responses to exercise. Varanids, known for their exceptional aerobic abilities among non-crocodylian sauropsids, can consume oxygen at rates twice as high as other similar-sized lizards (Thompson & Withers 1997; Farmer 2015a). This high level of aerobic activity is supported by their specialized heart structure, which includes thicker walls and a muscular ridge that effectively separates the chambers during heart-beat (Webb *et al.* 1971). The gular pump likely assists with ventilation in these lizards, supporting their capacity for high aerobic activity by ensuring balanced and compensated circulatory convection (Frappell *et al.* 2002). Moreover, unidirectional airflow without extrapulmonary air sacs has been identified in non-avian sauropsids such as crocodylians, varanids and iguanas (Farmer 2015b; Farmer & Sanders 2010; Schachner *et al.* 2014; Cieri *et al.* 2014). Crocodylians shares some cardiac features with the highly active birds like the complete septation between the atria and ventricles and the persistence of the right aortic arch as the parent vessel of the carotid arteries. However, unlike birds, both aortic arches persist in adult crocodylians. Although these animals are capable of sustained locomotion, they have a low gas transport capacity and require from 2 to 4 hours to recover after intense exercise (Bennett *et al.* 2019).

1985). The findings that notosuchians possessed slightly higher maximal metabolic rates than their extant crocodylian relatives show that a broader metabolic flexibility was present within crocodyliforms evolutionary history. This variation in the metabolic rates of notosuchians is in line with the countless studies that deal with the morphological and dental variation of this group, reinforcing the notion of an evolutionary capacity to occupy different ecological roles.

## Conclusions

This investigation into the metabolic rates of notosuchian crocodyliforms, which expands a previous dataset with nine additional tetrapod taxa, provided new and better insights into their physiological constraints. These findings confirm that notosuchians possessed a mass-independent maximal aerobic capacity that, although lower than that of varanid lizards, exceeded that of extant crocodylians. Comparatively, this elevated aerobic capacity likely facilitated a more active lifestyle, supported by their anatomical adaptations for terrestrial mobility, such as a parasagittal stance. The corrected MMR values for *Crocodylus porosus*, along with the inclusion of additional crocodylian species (i.e. *A. mississippensis* and *C. niloticus*) and an adjustment in the mass-independent MMR, have refined our understanding of notosuchian metabolic rates. This provides a clearer scenario of notosuchian ecological roles and adaptive strategies within the Cretaceous ecosystems.

## Supplementary Material

File 1. Dataset

File 2. Mass-independent_MMRdata1_0.84a

File 3. Mass-independent_MMRdata1_0.84b

File 4. Script_PEM_Notosuchia_2

*Acknowledgements.*— We thank R. Costa da Silva, curator of Palaeontology at the Museu de Ciências da Terra (MCTer, CPRM), for granting us access to the studied material. This research received support from: the European Research Executive Agency through the Marie Skłodowska-Curie Actions (MSCA) Postdoctoral Fellowship under the FLAPs Project (n° 101107135 to M. Sena); the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) with finance code 001 for funding (PROBRAL 88881.628047/2021-01 to F. Montefeltro); the Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq (CNPq #307922/2023-5 to F. Montefeltro), (CNPq #406902/2022-4 INCT PALEOVERT to G. Oliveira and J. Sayão), (CNPq# 309245/2023-0 to J. Sayão) and (CNPQ #308900/2021-9 to T. Marinho); and the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Rio de Janeiro – FAPERJ (E-26/210.066/2023 to J. Sayão). A. Pinheiro is grateful for the funds provided by the CNPq (MCTI/CNPq 28/2018 #434690/2018-0), which facilitated fieldwork that provided the specimens of *Coronelsuchus civali*.
References

Allan, G.H., Cassey, P., Snelling, E.P., Maloney, S.K., & Seymour, R.S. 2014: Blood flow for bone remodelling correlates with locomotion in living and extinct birds. *Journal of Experimental Biology* 217, 2956–2962.

Andrade, M.B., & Bertini R.J. 2008: A new *Sphagesaurus* (Mesoeucrocodylia: Notosuchia) from the upper Cretaceous of Monte Alto city (Bauru Group, Brazil), and a revision of the Sphagesauridae. *Historical Biology* 20, 101–136.

Aubier, P., Jouve, S. Schnyder, J. & Cubo, J. 2023: Phylogenetic structure of the extinction and biotic factors explaining differential survival of terrestrial notosuchians at the Cretaceous–Palaeogene crisis. *Palaeontology*, e12638.

Bennett, A.F. & Dawson, W.R. 1976: Metabolism. In Gans, C. & Dawson, W.R. (eds): *Biology of the Reptilia*, Vol. 5, pp. 127–224. Academic Press, London.

Bennett, A.F. 1978: Activity metabolism of the lower vertebrates. *Annual Review of Physiology* 40, 447–469.

Bennett, A.F., & John-Alder, H.B. 1984: The effect of body temperature on the locomotory energetics of lizards. *Journal of Comparative Physiology B* 155, 21–27.

Bennett, A.F., Seymour, R.S., Bradford, D.F. & Webb, G.J.W. 1985: Mass-dependence of anaerobic metabolism and acid-base disturbance during activity in the salt-water crocodile, *Crocodylus porosus*. *Journal of Experimental Biology* 118, 161–171.

Borsoni B.T., Carvalho I.S., & Marinho T.S. 2024: *Armadillosuchus arrudai* (Sphagesauridae, Crocodyliformes), Adamantina Formation (Turonian-Santonian), Bauru Basin, southeastern Brazil: Dental development aspects. *Cretaceous Research* 158, 105838.

Buffrénil V. de, Laurin M. & Jouve S. 2021: The crocodylomorpha. In: *Vertebrate Skeletal Histology and Paleohistology*. CRC Press, 486–510.

Carrier D.R. 1987: The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint. *Paleobiology* 13, 326–341.

Cieri, R.L., Craven, B.A., Schachner, E.R. & Farmer, C.G. 2014: New insight into the evolution of the vertebrate respiratory system and the discovery of unidirectional airflow in iguana lungs. *Proceedings of the National Academy of Sciences of the United States of America* 111, 17218–17223.

Cotts, L., Pinheiro, A.E.P., Marinho, T.S., Carvalho, I.S. & Di Dario, F. 2017: Postcranial skeleton of *Campinasuchus dinizi* (Crocodyliformes, Baurusuchidae) from the Upper Cretaceous of Brazil, with comments on the ontogeny and ecomorphology of the species. *Cretaceous Research* 70, 163e188.

Crispin, T.S. 2014: *The Evolution of Metabolic Rate in Terrestrial Ectotherms*. Unpublished PhD thesis, The University of Queensland, 205 pp.

Cubo, J., Sena, M.V.A., Pellarin, R., Faure-Brac, M.G., Aubier, P., Jouve, S., Allain, R. & Jalil, N.-E. 2024: Integrative paleophysiology of the metriorhynchoid *Pelagosaurus typus* (Pseudosuchia, Thalattosuchia). *The Anatomical Record*, 1–18. https://doi.org/10.1002/ar.25548

Cubo, J., Aubier, P., Faure-Brac, M.G., Martet, G., Pellarin, R., Pelletan, I. & Sena, M. 2023: Paleohistological inferences of thermometabolic regimes in Notosuchia (Pseudosuchia: Crocodylomorpha) revisited. *Paleobiology* 49, 342–352.

Cubo, J., Sena, M.V.A., Aubier, P., Houee, G., Claisse, P., Faure-Brac, M.G., Allain, R., Andrade, R.C.L.P., Sayão, J.M. & Oliveira, G.R. 2020: Were Notosuchia (Pseudosuchia: Crocodylomorpha) warm-blooded? A palaeohistological analysis suggests ectothermy. *Biological Journal of the Linnean Society* 131, 154–162.

Daniels, C., McGregor, L. & Nicholas, T. 1994: The dragon's breath: a model for the dynamics of breathing and faveolar ventilation in agamid lizards. *Herpetologica* 50, 251–261.

Farmer, C.G., & Hicks, J.W. 2000: Circulatory impairment induced by exercise in the lizard *Iguana iguana*. *Journal of Experimental Biology* 203, 2691–2697.

Farmer, C.G. 2015a: The Evolution of unidirectional pulmonary airflow. *Physiology* 30, 260–72.

Farmer, C.G. 2015b: Similarity of crocodilian and avian lungs indicates unidirectional flow is ancestral for archosaurs. *Integrative and Comparative Biology* 55, 962–971.

Farmer, C.G. & Sanders, K. 2010: Unidirectional airflow in the lungs of alligators. *Science* 327, 338–340.

Fernández Dumont M.L., Bona P., Pol D., Apesteguía S. 2020: New anatomical information on *Araripesuchus buitreraensis* with implications for the systematics of Uruguaysuchidae (Crocodyliformes, Notosuchia). *Cretaceous Research* 113, 104494.

Frappell, P., Schultz, T., & Christian, K. 2002: Oxygen transfer during aerobic exercise in a varanid lizard *Varanus mertensi* is limited by the circulation. *The Journal of Experimental Biology* 205, 2725–2736.

Gans, G. & Dawson, W.R. 1976: *Biology of the Reptilia. Volume 5, Physiology A*. Academic Press, London, New York & San Francisco.

Gearty, W. & Payne, J.L. 2020: Physiological constraints on body size distributions in Crocodyliformes. *Evolution* 74, 245–255.

Godoy, P.L., Bronzati, M., Eltink, E., Marsola, J.C., Cidade, G.M., Langer, M.C. & Montefeltro, F.C. 2016: Postcranial anatomy of *Pissarrachampsa sera* (Crocodyliformes, Baurusuchidae) from the Late Cretaceous of Brazil: insights on lifestyle and phylogenetic significance. *PeerJ* 26, e2075.

Guénard, G., Legendre, P., & Peres-Neto, P. 2013: Phylogenetic eigenvector maps: A framework to model and predict species traits. *Methods in Ecology and Evolution* 4, 1120–1131.

Hayes J.P., Feldman C.R. & Araújo M.B. 2018: Mass-independent maximal metabolic rate predicts geographic range size of placental mammals. *Functional Ecology* 32, 1194–1202.

Hu, Q., Nelson, T.J., & Seymour, R.S. 2021: Morphology of the nutrient artery and its foramen in relation to femoral bone perfusion rates of laying and non-laying hens. *Journal of Anatomy* 240, 94–106.

Knaus, P.L., Van Heteren, A.H., Lungmus, J.K. & Sander, P.M. 2021: Higher blood flow into the femur indicates elevated aerobic capacity in synapsids since the reptile-mammal split. *Frontiers in Ecology and Evolution* 2021, 751238.

Langer, M.C., Delcourt, R., Montefeltro, F.C., Silva Junior, J.C., Soler, M.G., Ferreira, G.S. & Batezelli, A. 2022: The Bauru Basin in São Paulo and its tetrapods. *Derbyana* 43, 776.

Larsson, H.C.E., & Sues, H.-D. 2007: Cranial osteology and phylogenetic relationships of *Hamadasuchus rebouli* (Crocodyliformes: Mesoeucrocodylia) from the Cretaceous of Morocco. *Zoological Journal of the Linnean Society* 149, 533–567.

Leardi, J. M., Pol, D., Montefeltro, F., Marinho, T.S., Ruiz, J.V., Bravo, G.G., Pinheiro, A.E.P., Godoy, P.L., Nicholl, C.S.C., Lecuona, A. & Larsson, H.C.E. 2024: Phylogenetic nomenclature of Notosuchia (Crocodylomorpha; Crocodyliformes). *Bulletin of Phylogenetic Nomenclature* 1, 044–082.

Maddison, W.P. & Maddison, D.R. 2014: *Mesquite: a Modular System for Evolutionary Analysis*. Available online at: http://mesquiteproject.org

Martinelli, A.G., Basilici, G., Fiorelli, L.E., Klock, C., Karfunkel, J., Diniz, A.C., Soares, M.V.T., Marconato, A., da Silva, J.I., Ribeiro, L.C.B. & Marinho, T.S. 2019: Palaeoecological implications of an Upper Cretaceous tetrapod burrow (Bauru Basin; Peirópolis, Minas Gerais, Brazil). *Palaeogeography, Palaeoclimatology, Palaeoecology* 528, 147–159.

Martins, K.C., Queiroz, M.V.L., Ruiz, J.V., Langer, M.C. & Montefeltro, F.C. 2024: A new Baurusuchidae (Notosuchia, Crocodyliformes) from the Adamantina Formation (Bauru Group, Upper Cretaceous), with a revised phylogenetic analysis of Baurusuchia. *Cretaceous Research* 153, 105680.

Melstrom K.M. & Irmis R.B. 2019: Repeated evolution of herbivorous crocodyliforms during the Age of Dinosaurs. *Current Biology* 29, 2389–2395.e3.

Milsom, W.K., Andrade, D.V., Brito, S.P., Toledo, L.F., Wang, T. & Abe, A.S. 2008: Seasonal changes in daily metabolic patterns of tegu lizards (*Tupinambis merianae*) placed in the cold (17 °C) and dark. *Physiological and Biochemical Zoology* 81, 165–75.

Montefeltro, F.C. 2019: The osteoderms of baurusuchid crocodyliforms (Mesoeucrocodylia, Notosuchia). *Journal of Vertebrate Paleontology* 39, e1594242.
Montefeltro, F.C., Lautenschlager, S., Godoy, P.L., Ferreira, G.S. & Butler, R.J. 2020: A unique predator in a unique ecosystem: modelling the apex predator from the Late Cretaceous crocodyliform-dominated fauna in Brazil. *Journal of Anatomy* 237, 323–333.

Munns, S. & Daniels, C. 2007: Breathing with big babies: ventilation and oxygen consumption during pregnancy in the lizard *Tiliqua rugosa*. *Physiological and Biochemical Zoology* 80, 35–45.

Nascimento, P.M. & Zaher, H. 2010: A new species of *Baurusuchus* (Crocodyliformes, Mesoeucrocodylia) from the Upper Cretaceous of Brazil, with the first complete postcranial skeleton described for the family Baurusuchidae. *Papéis Avulsos de Zoologia* 50, 323–361.

Nascimento, P.M. & Zaher, H. 2012: The skull of the Upper Cretaceous baurusuchid crocodile *Baurusuchus albertoi* Nascimento & Zaher 2010, and its phylogenetic affinities. *Zoological Journal of the Linnean Society* 163, S116–S131.

O'Connor, P., Sertich, J., Stevens, N., Roberts, E.M., Gottfried, M.D., Hieronymus, T.L., Jinnah, Z., Ridgely, R., Ngasala, S.E. & Temba, J. 2010: The evolution of mammal-like crocodyliforms in the Cretaceous Period of Gondwana. *Nature* 466, 748–751.

Ösi, A. 2013. The evolution of jaw mechanism and dental function in heterodont crocodyliforms. *Historical Biology* 26, 279–414.

Pinheiro, A.E.P., Pereira, P.V.L.G.C., Souza, R.G., Brum, A.S., Lopes, R.T., Machado, A.S., Bergqvist, L. & Simbras, F.M. 2018: Reassessment of the enigmatic crocodyliform 'Goniopholis paulistanus' Roxo, 1936: Historical approach, systematic, and description by new materials. *PLoS One* 13, e0199984.

Pinheiro, A.E.P., Souza, L.G., Bandeira, K.L.N., Brum, A.S., Pereira, P.V.L.G.C., Castro, L.O.R., Ramos, R.R.C. & Simbras F.M. 2021: The first notosuchian crocodyliform from the Araçatuba Formation (Bauru Group, Paraná Basin), and diversification of sphagesaurians. *Annals of the Brazilian Academy of Sciences* 93 (Supplement 2), e20201591.

Pol, D. 2005: Postcranial remains of *Notosuchus terrestris* Woodward (Archosauria: Crocodyliformes) from the Upper Cretaceous of Patagonia, Argentina. *Ameghiniana* 42, 21–38.

Pol, D., Leardi, J.M., Lecuona, A. & Krause, M. 2012. Postcranial anatomy of *Sebecus icaeorhinus* (Crocodyliformes, Sebecidae) from the Eocene of Patagonia. *Journal of Vertebrate Paleontology* 32, 328–354.

Pol, D., Nascimento, P.M., Carvalho, A.B., Riccomini, C., Pires-Domingues, R.A. & Zaher, H. 2014: A New Notosuchian from the Late Cretaceous of Brazil and the Phylogeny of Advanced Notosuchians. *Plos One* 9, e93105.

Pontzer, H., Allen, V., & Hutchinson, J.R. 2009: Biomechanics of Running Indicates Endothermy in Bipedal Dinosaurs. *Plos One* 4, https://doi.org/10.1371/journal.pone.0007783

Pyron, R.A., Burbrink, F.T. & Wiens, J.J. 2013: A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. *BMC Evolutionary Biology* 13, 1–53.

R Development Core Team 2023: *R: A Language and Environment for Statistical Computing*. R Foundation for Statistical Computing.

Ruiz, J.V., Queiroz, M.V.L., Martins, K.C., Godoy, P.L., Iori, F.V., Langer, M.C., & Montefeltro, F.C. & Bronzati, M. 2024: A new Peirosauridae (Crocodyliformes, Notosuchia) from the Adamantina Formation (Bauru Group, Late Cretaceous), with a revised phylogenetic analysis of Sebecia. *The Anatomical Record*, 1–24. https://doi.org/10.1002/ar.25559

Ruiz, J.V., Bronzati, M., Ferreira, G.S., Martins, K.C., Queiroz, M.V., Langer, M. & Montefeltro, F. 2021: A new species of *Caipirasuchus* (Notosuchia, Sphagesauridae) from the late Cretaceous of Brazil and the evolutionary history of Sphagesauria. *Journal of Systematic Palaeontology* 19, 265–287.

Schachner, E.R., Cieri, R.L., Butler, J.P. & Farmer, C.G. 2014: Unidirectional pulmonary airflow patterns in the savannah monitor lizard. *Nature* 506, 367–370.

Sena, M.V.A., Montefeltro, F.C., Marinho, T.S., Langer, M.C., Fachini, T.S., Piacentini Pinheiro, A.E., Machado, A.S., Lopes, R.T., Pellarin, R., Sayão, J.M., Oliveira, G.R. & Cubo, J. 2023: The cost of living in Notosuchia (Crocodyliformes, Mesoeucrocodylia). *Palaeogeography, Palaeoclimatology, Palaeoecology* 632, 111855.

Sena, M.V., Andrade, R.C., Sayão, J.M. & Oliveira, G.R. 2018: Bone microanatomy of *Pepesuchus deiseae* (Mesoeucrocodylia, Peirosauridae) reveals a mature individual from the Upper Cretaceous of Brazil. *Cretaceous Research* 90, 335–348.

Seymour, R.S., Hu Q., Snelling, E.P. & White, C.R. 2019: Interspecific scaling of blood flow rates and arterial sizes in mammals. *Journal of Experimental Biology* 222, jeb199554.

Seymour, R.S., Gienger, C.M., Brien, M.L., Tracy, C.R., Manolis, S.C. & Christian, K.A. 2013: Scaling of standard metabolic rate in estuarine crocodiles *Crocodylus porosus*. *Journal of Comparative Physiology B* 183, 491–500.

Seymour, R.S. 2013: Maximal aerobic and anaerobic power generation in large crocodiles versus mammals: implications for dinosaur gigantothermy. *PLoS One* 8, e69361.

Seymour, R.S., Smith, S.L., White, C.R., Henderson, D.M. & Schwarz-Wings, D. 2012: Blood flow to long bones indicates activity metabolism in mammals, reptiles, and dinosaurs. *Proceedings of the Royal Society B: Biological Sciences* 279, 451–456.

Seymour, R.S., Caldwell, H.R., Woodward, H.N. & Hu, Q. 2024: Growth rate affects blood flow rate to the tibia of the dinosaur *Maiasaura*. *Paleobiology* 50, 123–129.

Thompson, G.G. & Withers, P.C. 1997: Standard and maximal metabolic rates of goannas (Squamata: Varanidae). *Physiological Zoology* 70, 307–323.

Upham, N.S., Esselstyn, J.A. & Jetz, W. 2019: Inferring the mammal tree: Species-level sets of phylogenies for questions in ecology, evolution, and conservation. *PLoS Biology* 17, e3000494.

Varela, L., Tambusso, S. & Farina, R. 2024: Femora nutrient foramina and aerobic capacity in giant extinct xenarthrans. *PeerJ* 1–25.

Vasconcellos, F.M. & Carvalho, I.S. 2006: Condicionante etológico na tafonomia de *Uberabasuchus terrificus* (Crocodyliformes, Peirosauridae) da Bacia Bauru (Cretáceo Superior). *Geociências* 25, 225–230.

Vidal, N., & Hedges, S.B. 2005: The phylogeny of squamate rep-tiles (lizards, snakes, and amphisbaenians) inferred from nine nuclear protein-coding genes. *Comptes Rendus Biologies* 328, 1000–1008.

Villa, A., Abella, J., Alba, D.M., Almécija, S., Bolet, A., Koufos, G.D., Knoll, F., Luján, A.H., Morales, J., Robles, J.M., Sánchez, I.M. & Delfino, M. 2018: Revision of *Varanus marathonensis* (Squamata, Varanidae) based on historical and new material: Morphology, systematics, and paleobiogeography of the European monitor lizards. *PLoS One* 13, e0207719.

Wang T., Warburton S., Abe A. & Taylor T. 2001: Physiological Society Symposium — Vagal control: from Axolotl to man. *Experimental Physiology* 86, 777–784.

Webb, G., Heatwole, H. & De Bavay, J.D. 1971: Comparative cardiac anatomy of the reptilia. I. The chambers and septa of the varanid ventricle. *Journal of Morphology* 134, 335–350.

Weibel, E.R., Bacigalupe, L.D., Schmitt, B. & Hoppeler H. 2004: Allometric scaling of maximal metabolic rate in mammals: muscle aerobic capacity as determinant factor. *Respiratory Physiology & Neurobiology* 140, 115–132.

White, C.R. & Seymour, R.S. 2005: Allometric scaling of mammalian metabolism. *The Journal of Experimental Biology* 208, 1611–1619.

Woodward, H.N., Aubier, P., Sena, M.V.A. & Cubo, J. 2024: Evaluating extinct pseudosuchian body mass estimates using a femur volume-based model. *The Anatomical Record*. https://doi.org/10.1002/ar.25452.

Zurano, J.P., Magalhaes, F.M., Asato, A.E., Silva, G., Bidau, C.J., Mesquita, D.O. & Costa, G.C. 2019: Cetartiodactyla: Updating a time-calibrated molecular phylogeny. *Molecular Phylogenetics and Evolution* 133, 256–262.