Sparassocynus

Beck, Robin M. D., Voss, Robert S. & Jansa, Sharon A., 2022, Craniodental Morphology And Phylogeny Of Marsupials, Bulletin of the American Museum of Natural History 2022 (457), pp. 1-353 : 316-317

† 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.