A large carnivorous mammal from the Late Cretaceous and the North American origin of marsupials

Marsupial mammal relatives (stem metatherians) from the Mesozoic Era (252–66 million years ago) are mostly known from isolated teeth and fragmentary jaws. Here we report on the first near-complete skull remains of a North American Late Cretaceous metatherian, the stagodontid Didelphodon vorax. Our phylogenetic analysis indicates that marsupials or their closest relatives evolved in North America, as part of a Late Cretaceous diversification of metatherians, and later dispersed to South America. In addition to being the largest known Mesozoic therian mammal (node-based clade of eutherians and metatherians), Didelphodon vorax has a high estimated bite force and other craniomandibular and dental features that suggest it is the earliest known therian to invade a durophagous predator–scavenger niche. Our results broaden the scope of the ecomorphological diversification of Mesozoic mammals to include therian lineages that, in this case, were linked to the origin and evolution of marsupials.


Dental measurements (Supplementary
Dental measurements were taken using a Leica MZ9.5 binocular dissecting microscope and custom measuring stage capable of reading to 0.001 mm. The parietal appears to contact the alisphenoid anterolaterally, as is the case in most didelphids 29 , the squamosal laterally, and the interparietal posteriorly.

Nasal-In
Interparietal-In dorsal view, the lambdoidal crest is posteriorly convex with an anterior emargination at the midline. Subtle differences in bone texture and a suture with the supraoccipitals ventral to the lambdoidal crest implies the presence of an interparietal bone, as in all didelphids and some non-didelphid marsupials 29 . The interparietal appears to contact the squamosal laterally, as is the case in some didelphids (e.g., Didelphis) but not in most nondidelphid marsupials 29 . Figs. 1e and 2e), the nasopharygneal region is broad and U-shaped anteriorly, rather than posteriorly tapering as in Didelphis virginiana. The pterygoids appear to be partially preserved, but the contact with the medial surface of the perpendicular process of the palatine is unclear.

Pterygoid-In ventral view of NDGS 431 (Supplementary
Presphenoid-A splint-like pre-sphenoid is preserved. The tympanic process of the alisphenoid contributes to the bulla in several extinct and extant metatherians 23,44,47,[52][53][54] , suggesting that the structure is plesiomorphic for Marsupialia. However, the bulla is absent in some early Paleocene marsupialiforms, including Pucadelphys and Mayulestes 41,50,51 , leading to the supposition that the alisphenoid contribution to the bulla evolved more than once within Metatheria. However, that hypothesis was based in part on the hypothesized absence of a bulla in stagodontids such as Didelphodon. Now that a complete bulla is known for Didelphodon, it is more parsimonious that the alisphenoid bulla is ancestral for metatherians and secondarily lost in Pucadelphys and Mayulestes, as suggested by Kielan-Jaworowska and Nessov 52 .

Petrosal-The generalized petrosal bone of therian mammals consists of the anterior pars
cochlearis, which houses the cochlear canal containing the spiral organ of hearing, and the pars canalicularis, which houses the organs of balance and equilibrium within the bony vestibule, semicircular canals, and associated ampullae. Much of the anatomy of the petrosal confirms the morphology described by Clemens 45 . In ventral aspect, the left petrosal is shielded from view by the tympanic bulla ( Supplementary Fig. 7). The left bulla was digitally removed from 3D reconstructions skull rendered from CT data to reveal that the ventral surface of the pars cochlearis is damaged, but the pars canalicularis is more-or-less complete. The right petrosal is exposed and easily visible without the use of CT imaging. Although the pars cochlearis is complete on the right side of the skull, the pars canalicularis is damaged and the ventrally projecting mastoid process is missing.
The most distinctive feature of the pars cochlearis in ventral view is the promontorium containing the cochlea itself ( Supplementary Fig. 8). The promontorium is rounded posteroventrally, and the bone is fairly thin ventral to the internal cochlear cavity, but thick medial, lateral, anterior, and dorsal to the bony labyrinth, as revealed by the CT data ( Supplementary Fig. 7b) Fig. 8c-d). A similar groove with partial canal was described previously for an isolated petrosal assigned to Didelphodon vorax (UCMP 52869), and the occupant of the sulcus was identified as a branch of the internal carotid artery at that time (Clemens 45 , following Patterson 56 ). However, the sulcus in NDGS 431 is in the same location and of similar morphology as the sulcus that transmits the inferior petrosal sinus in extant marsupials, such as Didelphis virginiana 57 (see also Archibald 58 ), and a sulcus for the internal carotid artery leading to the anterior carotid foramen is present anterior and external to the tympanic bulla as described above (Supplementary Fig. 7). Thus, we identify the medial groove as the sulcus for the inferior petrosal sinus.
A broad and distinct sulcus that contributes to the posttemporal canal for the diploëtic artery and vein (arteria and vena diploetica magna) is observed lateral to the subarcuate fossa, extending in an anterior-posterior direction along the lateral surface of the petrosal (Supplementary Fig. 9a).
A corresponding sulcus on the squamosal completes a canal between the two bones ( Supplementary Fig. 7b). Anterior to the petrosal, the canal curves ventrally and splits into an Dimensions of bony labyrinth of NDGS 431 are provided in Supplementary Table 1. The cochlea at the anteroventral end of the bony labyrinth contributes less than half of the total inner ear volume (46.1%). The cochlea is better preserved on the right side of the skull, and the canal completes just over one and three quarters turns ( Supplementary Fig. 10). The degree of coiling measured here for Didelphodon (1.8 turns) is greater than that measured for any other Mesozoic therian described to date [64][65][66][67][68][69] . Although the apical turn of the cochlea is poorly preserved within the left petrosal (on account of damage to the ventral surface of the promontorium), it appears that the left cochlea coils to the same degree as the right. The cochlea has a low profile compared to more recent mammals 70 with an aspect ratio of 0.42 for the left cochlea. Neither the right nor the left cochlea is expanded at its apical end indicating a lagena, and no canals for lagenar nerves were observed.
Internal structures within the cochlea, such as the primary and secondary bony laminae to support the basilar membrane, as well as the Rosenthal's canal for the spiral nerve ganglion, were difficult to resolve in the CT data on account of the delicate nature of the structures.
Likewise, presence of a cribriform plate for individual auditory nerve fibers within the internal auditory meatus could not be resolved in CT images, but likely are present given their occurrence in more basally diverging cladotherians 71 . Traces of the secondary bony lamina are visible on the endocasts as a faint groove along the radial (outer) wall of the cochlea, particularly in the left labyrinth ( Supplementary Fig. 10a-c), but the robustness and total extent of the structure could not be determined. The bony canaliculus cochleae for the membranous perilymphatic duct is complete and undamaged on the right side of the skull and takes a straight course as it exits the cochlear cavity ( Supplementary Fig. 10b, d). The left petrosal is fractured across the course of the canaliculus, giving the endocast of the canaliculus a bent appearance on that side of the skull ( Supplementary Fig. 10a, c).  Table 2). The three semicircular canals are more or less planar, although the central course of the anterior canal is slightly distorted, which gives the anterior canal an appearance of torsion ( Supplementary Fig. 10c-d). The distortion is the result of preservation (the rim of the subarcuate fossa has been crushed and pushed in towards the center of the fossa in that region) rather than biological torsion of the canal from its plane.  Table 2).
The anterior semicircular canal is larger than either the lateral or posterior canals in terms of length of the slender (unampullated) portion of the canal and canal arc radius (Supplementary Table 1 The alveolar row slopes dorsally into the premaxillary groove, which received the lower canine.
In UWBM 94084 ( Supplementary Figs. 3 and 4), right C1 is unworn except for slight apical wear and a narrow facet along the length of the crown on the mesial aspect, which was formed by attritional wear with the opposing lower canine. The crown is tall and robust (see Supplementary Table 9 in NDGS 431, it terminates mesially in two small cuspules, but those cuspules are not preserved in SCNHM VMMa 20. The longer, more gently sloping distal ridge also terminates distally with a cuspule. From that distal cuspule a ridge extends toward the lingual lobe, forming a shallow basin between the distal ridge and the distolingual ridge. Lingual to this shallow basin, the lingual lobe is gently sloping and has an inflated appearance. The P2 is bilobate with a large buccal cusp and smaller lingual cusp that merge into flat, circular facets with wear ( Supplementary Figs. 5d and 6d). Its mesial concavity receives the slightly convex distal aspect of P1. A short distal ridge does extend from the main buccal cusp to a distobuccal cuspule. A cuspidate distolingual ridge extends from this distobuccal cuspule to the lingual cusp, forming a basin that is shallower than that in P1. The lingual lobe is mesiodistally broad, symmetrical and has an inflated appearance that overhangs the crown cervix.
The P3 is rectangular in occlusal outline. In UWBM 94084 and to a lesser degree in UWBM 94500, the P3 crown is nearly worn flat to the cervix. In SCNHM VMMa 20 and NDGS 431, the P3s are relatively unworn and are taller than all other cheek teeth. The buccal cusp is tall with a prominent distal ridge that terminates in a cuspule; a distolingual ridge extends from that cuspule onto the broad, bulging lingual lobe. There is a less distinct mesial cusp and shorter, subtler mesial and mesiolingual ridges. In size, P3>P2>P1. The enamel on all of the premolars is vertically corrugated, particularly on the buccal and lingual faces of the main cusp. On P2 and P3 below the cervix, the lingual root has a roughened and bulging appearance, presumably from deposition of cement 75,76 .

Supplementary Note 2: Phylogenetic character list and scores for Didelphodon
Below is the character list and descriptions; characters 1-156 are from Rougier et al. 77  103. Orbitotemporal canal -present (0) or absent (1).

Supplementary Note 4: Templeton tests of Alternative Topologies #1 and #2
We submitted our data matrix with a single positive force constraint (Alternative Topology #1: 'Gurlin Tsav skull', SA stem marsupialiforms, and Marsupialia) to a traditional search in TNT (version 1.1 80 ), using 100 replicates and 10,000 saved trees per replication. We obtained 28 EPTs with tree length = 605. The consensus tree is shown below.    We submitted our data matrix with a single positive forced constraint (Alternative Topology #2: SA stem marsupialiforms and Marsupialia) to a traditional search in TNT (version 1.1 80 ), using 100 replicates and 10,000 saved trees per replication. We obtained 22 EPTs with tree length = 603. The consensus tree is shown below.
tree) versus the above tree with the Alternative Topology #2 (SA stem marsupialiforms + Marsupialia). The following is the output.