Sparassocynus undetermined
Creators: name: Beck, Robin M. D.
name: Voss, Robert S.
name: Jansa, Sharon A.
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Didelphimorphia
Didelphidae
Sparassocynus
Sparassocynus undetermined

description: † Sparassocynus   
 SPECIES SCORED: † Sparassocynus bahiai (type species), † S. derivatus.   
 GEOLOGICAL PROVENANCE OF SCORED SPECIMENS: All examined specimens of † Sparassocynus bahiai were collected in the vicinity of Monte Hermoso in Buenos Aires Province, Argentina, presumably from the coastal Monte Hermoso Formation (Farola Monte Hermoso; Tomassini et al., 2013). Examined specimens of † S. derivatus are from the Chapadmalal Formation, which is exposed along the coastline between Miramar and Mar del Plata, also in Buenos Aires Province, Argentina (Isla et al., 2015).   
AGE OF SCORED SPECIMENS: The fossil-bearing sediments near Monte Hermoso are the type locality of the Montehermosan South American Land Mammal Age (SALMA) and are estimated to be younger than 5.28 ± 0.04 Mya based on radiometric dating of impact glasses and older than 4.5 or 5.0 Mya based on magnetostratigraphy (Zárate, 2005; Schultz et al., 2006; Tomassini et al., 2013). Impact glasses from the top of the Chapadmalal Formation, type locality of the Chapadmalalan SALMA, yield radiometric dates of 3.3 ± 0.2 Mya (Schultz et al., 1998), a plausible minimum age for the material we examined from this formation, for which Woodburne (2010: fig. 3) suggested a maximum age of about 5 Mya (see also Prevosti and Forasiepi, 2018: table 1.1).  
ASSIGNED AGE RANGE: 5.320 –3.100 Mya.  
 REMARKS: Originally described by Mercerat (1899) as a sparassodont based on fragmentary dental remains from the Monte Hermoso Formation (see also Ameghino, 1899: 7; Cabrera, 1927: 306), † Sparassocynus was later referred to the didelphid subfamily Sparassocyninae by Reig (1958b). This arrangement was maintained by Reig and Simpson (1972) in their description of additional well-preserved cranial material of the genus. However, Reig et al. (1987) and several subsequent authors (e.g., Goin, 1991, 1995; Forasiepi et al., 2009; Abello et al., 2015) classified † Sparassocynus and a second fossil taxon, † Hesperocynus (see below) in a separate family, † Sparassocynidae, which they placed together with Didelphidae (sensu Voss and Jansa, 2009) in the superfamily Didelphoidea.   
 Beck and Taglioretti (2020) described the skull of a well-preserved, late-stage juvenile of † Sparassocynus derivatus (MMP M-5292, collected from the Chapadmalal Formation), which revealed the presence of a distinctive cranial apomorphy of Didelphidae (early fusion between the interparietal and supraoccipital; see char. 31), as well as other apparently apomorphic craniodental features characteristic of subclades within Didelphidae. The latter include a posterior palatal margin (postpalatal torus) with distinct “corners” (found in all Recent didelphids except Caluromys, Caluromysiops, and Glironia; Voss and Jansa, 2003; 2009), contact between the maxilla and alisphenoid in the ventral floor of the orbit (also present in Lutreolina, Monodelphis, and † Thylatheridium; Voss and Jansa, 2003, 2009; see char. 16), and fusion in subadults of the midfrontal suture (as found in Chironectes, Didelphis, Lutreolina, and Philander; Voss and Jansa, 2003, 2009; see char. 24). Beck and Taglioretti (2020) also reinterpreted the specialized, highly inflated auditory region of † Sparassocynus, documenting the presence of an enormously expanded hypotympanic sinus (part of which was identified as an epitympanic sinus by Reig and Simpson, 1972; see also comments by Forasiepi et al., 2009: 1256), a posterior squamosal epitympanic sinus (see char. 84), and an unusual course for the mandibular division of the trigeminal nerve (within a bony canal in the medial wall of the hypotympanic sinus; see char. 52). Of these, the last two also appear to be present in † Hesperocynus (see below), whereas the first cannot be assessed based on available material.   
 The total-evidence Bayesian phylogenetic analyses of Beck and Taglioretti (2020), which used a dataset modified from Voss and Jansa (2009), recovered † Sparassocynus and † Hesperocynus as sister taxa within Didelphidae, either inside the genus Monodelphis (in the undated analysis) or, perhaps more plausibly, sister to Monodelphis (in the tip-and-node dating analysis). Beck and Taglioretti (2020) argued that † Sparassocynus and † Hesperocynus warranted distinction at the tribal level within Didelphidae (as †Sparassocynini). The analyses presented here represent a further test of the phylogenetic relationships of † Sparassocynus and † Hesperocynus based on an expanded and revised character set and a wider sampling of marsupials and nonmarsupial metatherian outgroup taxa.   
 Although we scored † Sparassocynus based on craniodental material traditionally identified as † S. bahiai (from the Monte Hermoso Formation) and † S. derivatus (from the Chapadmalal Formation), these nominal species were distinguished only by Reig and Simpson (1972: 515) based on minor dental differences, and they do not differ in size (Abello et al., 2015); they could plausibly be synonymized.
description: Published as part of Beck, Robin M. D., Voss, Robert S. & Jansa, Sharon A., 2022, Craniodental Morphology And Phylogeny Of Marsupials, pp. 1-353 in Bulletin of the American Museum of Natural History 2022 (457) on pages 316-317, DOI: 10.1206/0003-0090.457.1.1, http://zenodo.org/record/6971356