Cifellilestes ciscoensis, Davis & Jäger & Rougier & Trujillo & Chamberlain, 2022

Davis, Brian M., Jäger, Kai R. K., Rougier, Guillermo W., Trujillo, Kelli & Chamberlain, Kevin, 2022, A morganucodontan mammaliaform from the Upper Jurassic Morrison Formation, Utah, USA, Acta Palaeontologica Polonica 67 (1), pp. 77-93 : 80-86

Cifellilestes ciscoensis sp. nov.

Zoobank LSID: urn:lsid:zoobank.org:act:801C5454-4299-4352-AB6E E78CC923810D

Etymology: In reference to the ghost town of Cisco, Utah, close to the type locality to which it also lends its name.

Holotype: OMNH 80538 , right skull fragment preserving partial palate and snout, and postcanine dentition.

Type locality: OMNH V1728 , Cisco Mammal Quarry , Grand County, Utah, USA .

Type horizon: Brushy Basin Member of Morrison Formation, Tithonian, Upper Jurassic.

Material.— Holotype and OMNH 69352 , left skull fragment preserving partial palate and snout, and postcanine dentition; from type locality and horizon.

Diagnosis.—Upper molariforms with three primary cusps arranged in a line, but differing from eutriconodontans and resembling morganucodontans in the presence of complete, cuspidate lingual and buccal cingula and the absence of interlock between adjacent molars; resembles species of Morganucodon but differs from other morganucodontans in pronounced imbrication of distal upper teeth; differs from known morganucodontans in presence of only two molars, molarization of distal three premolars, presence of a single, large infraorbital foramen, and of a buccinator ridge on the lateral alveolar margin of the maxilla.

Description.—Both specimens, left and right skull fragments, preserve a nearly complete maxilla and much of the palatine. Both specimens are broken in a strikingly similar manner with only minor differences in morphology ( Figs. 1 , 2 ); however, minor differences in size and degree of tooth wear preclude referral to the same individual. Both specimens are presumed to be adults based on the heavy wear of the ultimate molar and the absence of replacement teeth within the maxilla. OMNH 69352 is slightly larger than the holotype based on the length of the postcanine alveoli and shows a slightly more advanced stage of tooth wear (though some details of the pattern are not identical), suggesting this specimen represents an older individual. The dentition is better preserved in the holotype (OMNH 80538), and that specimen will be the focus of the description with differences in morphology between the two specimens noted where they occur. The well-preserved facets for the jugal and lacrimal as well as the three-dimensional preservation of the delicate orbital and pterygoid processes of the palatine (though damaged) suggest that these specimens were gently disarticulated from their adjacent skull elements during deposition ( Fig. 3 ). Some low-energy transport occurred as the specimens were isolated. While the referred specimen was identified as a mammal maxilla in the field, the holotype was only recognized after some mechanical preparation following the field season; we therefore have no record of how close to one another the specimens were recovered.

Lateral view: The maxilla appears to be nearly complete except for some breakage anteriorly in the vicinity of the canine alveolus and some likely minor breakage along the area interpreted as the suture with the nasal ( Fig. 4 ). The facial process of the maxilla is nearly vertical and convex dorsally, tapering in lateral view posteriorly to a low, laterally flaring base for the zygomatic arch and anteriorly to the presumed contact with the premaxilla (though the suture is not preserved in either specimen). The maxilla includes the entirety of the single large canine alveolus, and no incisor alveoli are preserved within the maxilla in contrast with the condition in Morganucodon watsoni Kühne, 1949 , and docodontans

Kermack et al. 1973; Lillegraven and Krusat 1991; Rougier et al. 2014). The dorsal portion of the facial process curves slightly medially, resulting in a tall rostrum only slightly rounded towards the presumed contact with the nasals. Most of the curved dorsal edge of the maxilla is interpreted as the natural margin of the suture with the nasal, and the shape of this margin suggests the nasals were wider posteriorly than anteriorly. The prong along the dorsal margin directly above the P4 is here interpreted as the position where contact occurred with the nasal anteriorly and the lacrimal posteriorly

Figs. 1A 1 , 4A 1 ). Low on the maxilla and stretching about the length of the two molars there is a well textured, oval facet in the maxilla that continues posteriorly following the overhang of the zygomatic process and the maxillary tuberosity. Most of the large, dorsolaterally-facing facet was likely occupied by the jugal, leaving only a small contact area for the lacrimal immediately dorsal to the infraorbital foramen. The maxilla continues posteriorly beyond the base of the zygoma, with the distal root of the ultimate molar fully posterior to the contact with the jugal. There is little bone posterior to this in either specimen, suggesting these are adults and no additional teeth would be expected. There is a single large, anteroposteriorly elongate infraorbital foramen, positioned dorsal to the mesial root of the P5 or between the P4 and P5

Figs. 1A 1 , 2A 1 , 4A 1 ). However, the medial and lateral walls of the foramen bear low ridges that give it a faint figure-eight shape in coronal section. The direction of the infraorbital canal and foramen suggest that the infraorbital branch of V 2 and accompanying vessels were directed anteriorly, high on the rostrum in the direction of the postcanine recess. Though the lateral wall of the canine alveolus is crushed, there is no indication of a large boss for the canine root, suggesting that this tooth was moderate in size. There are three small foramina on the lateral snout, one dorsal to the canine, one dorsal to the mesial root of the P1, and a third smaller one in between and ventral to these. These open into the alveolar canal and would likely have transmitted blood vessels and/or small branches of V 2, as in non-mammaliaform cynodonts (see discussion of the rostral alveolar canal and foramen in Benoit et al. 2016b, 2020). As these foramina are well removed anteriorly from the primary opening of the infraorbital canal, we do not consider them to represent additional infraorbital foramina. A distinct, thickened ridge contours dorsally along the rim of the alveolar process; this is the buccinator ridge, which is particularly distinct between the P1–M1. The size and orientation of the infraorbital foramen and the clear buccinator ridge indicate the presence of fleshy cheeks in Cifellilestes gen. nov. (as seen in extant mammals, e.g., Pietrokovski and Massler 1967).

Ventral view: The bony palate preserves contributions from the maxilla and palatine ( Fig. 4A 3 ). The palatal process of the maxilla is broken and displaced in some parts, but some portions of the midline suture are preserved. No portion of the incisive foramen is preserved. Though crushed laterally, the single canine alveolus appears to be complete or nearly complete and we interpret it was entirely housed within the maxilla. The zygomatic process of the maxilla is well exposed in ventral view with its lateral-most extent even with the mesial root of the M2; most of the crown of this tooth is posterior to the anterior root of the zygomatic arch. The maxillary tuberosity is present on the narrow rim of bone posterior to the M2. There are substantial pits present just medial to the roots of the posterior dentition to accommodate the protoconids of the lower teeth, which must have been conical and quite tall. The first and shallowest of these pits is positioned between the P3–P4, with the posteriormost between the molars also rather shallow. The pits between the P4–P5 and P5–M1 are very deep, with the latter the deepest. The maxilla–palatine suture is nearly transverse at its most medial point, even with the paracone of the P3. From there, the suture arcs posterolaterally just medial to the palatal pits. The palatine has been displaced by breakage in the holotype but is preserved in-situ in the referred specimen. The posterior margin of the bony palate curves medially to a prominent posterior nasal spine, positioned even with the M1–M2 boundary. It appears that the posterior end of the bony palate in the holotype may have been positioned slightly more anteriorly, perhaps a half-tooth length, though this is uncertain due to some displacement of the palatine due to breakage ( Figs. 1A 3 , 2A 3 ). The referred specimen is undistorted here, and the posterior margin of the bony palate reaches the anterior half of the ultimate molar ( Fig. 2A 3 ). The greater palatine foramen is positioned on the maxilla– palatine suture, at the level of the P4. A prominent groove continues anteriorly from this opening. Two lesser palatine foramina are present on the holotype, directed anteriorly on a horizontal plane and opening just medial to the distal root of the ultimate molar (only one lesser palatine foramen is present on the referred specimen). Only a thin bridge of bone encloses these foramina ventrally, and prominent grooves extend from them anteriorly on the palate. Posteriorly from the lateral margin of the palate, the palatine tapers before flaring into a club-shaped pterygoid process. The narrowed neck of this region of the palatine is laterally convex and medially concave, where it frames the margin of the choana. A thin, sub-horizontal plate of bone is present dorsally where the palatine would have met the sphenoid complex in the mesocranium. Posteriorly, the presumed contact with the pterygoid is complex; a thin vertical ridge extends posteriorly on the medial side, while a blunt, triangular knob is present more anteriorly and projects laterally. This knob supports a low crest and delimits a shallow pocket on the posterolateral surface of the pterygoid process, and we interpret this to be a complex articulation with what would have likely been a heavy pterygoid element or perhaps even the alisphenoid. While unclear due to preservation, the pterygoid ramus of the alisphenoid would likely have overlapped at least to some extent this club-shaped structure dorsally. This arrangement of the sidewall of the choanae is similar to that described for some close non-mammaliaform outgroups, such as Chaliminia musteloides Bonaparte, 1980 , and species of Massetognathus , among others ( Martinelli and Rougier 2007; Rowe and Shepherd 2016; Crompton et al. 2017).

Orbit: Very little of the orbit is preserved, only the contribution by the maxilla and part (presumably) of the orbital process of the palatine. The maxilla forms a horizontal shelf in the anterior floor of the orbit (the orbital platform), medial to the root of the zygoma. The roots of the molars do not pierce the maxillary platform and cannot be seen in the orbit. The posterior opening of the infraorbital canal, the maxillary foramen, is present on the orbital platform ( Fig. 4A 4 ); the dorsal rim of the canal was likely formed by the lacrimal but is not preserved. This canal is preserved in both specimens as a well-defined but dorsally open trough within the maxilla between the floor of the orbit and the opening onto the face, and is only briefly enclosed by the maxilla as it exits anteriorly. There are one or two small foramina on the maxillary platform posterior to the maxillary foramen that travel anteroventrally to communicate with the alveolar canal; these likely held small (perhaps middle alveolar) branches of V 2. The infraorbital canal communicates with the remaining anterior portion of the alveolar canal through a small passageway just prior to its exit onto the face (visible in CT slices). The preserved portion of the orbital process of the palatine is tall, extending vertically above the level of the presumed contact between the maxilla and nasal ( Fig. 4A 1 ). It forms a right angle in horizontal section, with a thin lateral process oriented towards the position of the lacrimal and a thin plate that would have formed a portion of the medial orbital wall. Breakage precludes any estimate of contact with other bones expected in this region, such as the frontal or orbitosphenoid, but the height of the orbital process suggests that the palatine formed a considerable portion of the medial wall of the orbit. The nasal cavity and the orbit were separated by a full bony wall that, in addition to the maxilla, a large palatine, and the lacrimal, likely also included the now missing frontal. Transmission of neurovascular structures between the two regions would have been accomplished via distinct, relatively small foramina as in modern mammals. A large, oval-shaped sphenopalatine foramen is preserved, opening into the nasal cavity medial to the base of the orbital process of the palatine ( Fig. 4A 2 ). The floor of this foramen in the referred specimen is divided into a pair of grooves. This foramen would have transmitted neurovasculature from V 2 (plus autonomic fibers from the greater petrosal branch of VII) and the maxillary artery to the nasal cavity and palate, as occurs in living mammals.

Nasal cavity: The nasal cavity is tall and broad posteriorly, and while seeming to retain much of its height anteriorly (though the frontals and nasals are not preserved), a narrowing of the snout anterior to the infraorbital foramen sharply reduces the width of the nasal cavity. A prominent oval pocket occupies the posterolateral floor of the nasal cavity, immediately anterior to the orbital process of the palatine. This is identified as the maxillary recess following Crompton et al. 2017). The maxilla–palatine suture is interpreted to pass obliquely across the posterior third of the maxillary recess. No turbinals are preserved. Three parallel grooves are visible coursing along the lateral wall of the nasal cavity ( Fig. 4A 2 ). The superior two of these likely represent attachment sites for maxilloturbinals, and the inferiormost groove was likely for the nasolacrimal duct. A thin but prominent transverse flange is preserved extending medially from near the base of the orbital process of the palatine ( Fig. 4A 2 ) (see terminology in Maier 1993). It passes parallel to the palatal process of the palatine; this shelf would likely have contacted the vomer, as in species of Morganucodon and close sister taxa ( Kermack et al. 1981; Crompton et al. 2017). Here, the transverse flange would delimit a rather shallow inferior air passage at the rear of the nasal cavity (which communicated through the choana into the pharynx) and also formed the floor of the sphenoidal recess at the posterodorsal extent of the nasal cavity. The sphenopalatine foramen opens anteromedially into the nasal cavity adjacent to the base of the transverse flange. A groove connects the foramen with the dorsal opening of the greater palatine foramen, which is positioned along the maxilla–palatine suture and directed anteriorly ( Fig. 4A 4 ). The infraorbital canal is interpreted to have lacked exposure to the nasal cavity. There is no evidence of a maxillary sinus developed in the substance of the maxilla.

Dentition: Upper dental formula is interpreted as?.1.5.2 see Table 1 for tooth measurements). No incisor alveoli are preserved in the maxilla though it is uncertain if the maxilla continued anteriorly from the preserved margin around the canine alveolus. The canine is represented by a single, large, elliptical alveolus ( Figs. 1A 3 , 4A 3 ); though crushed on both specimens, there is no indication that this alveolus was divided and root length and proportions compared with Morganucodon spp. suggest the tooth was single rooted. Five double-rooted premolars are present, all of which have a short crest bridging the roots at the base of the crown. None of the postcanine teeth show evidence of apical expansion of the roots, nor are the roots fused for any portion of their length. The referred specimen bears a distinct diastema between the canine and P1 ( Fig. 2A 3 ), and the alveolar bone in this region is textured and appears modified as would be expected if a tooth position had been shed without replacement. CT data appear similar but do not reveal evidence of alveoli or roots in this region, and the holotype lacks a diastema. We therefore interpret this as a variable presence of a C–P1 diastema, perhaps related to the age (based on tooth wear) and larger size of the referred specimen, instead of evidence of loss of a mesial premolar position. The premolars gradually increase in size and complexity distally ( Fig. 1B ).

Figure 5 illustrates our cusp nomenclature for the postcanine teeth, as outlined above in the Materials and conventions section. The P1 is small, simple, and premolariform, with a somewhat laterally compressed paracone and a much lower but still well-developed distal heel cusp; we identify this as the metacone. A faint bulge is present at the distalmost edge of the crown; this bulge transitions to a substantial cingular cusp through the premolar series and is here interpreted as the metastyle. A very faint bump is present on the mesial crown base; on distal teeth, we identify the cusp in this position as cusp B. The P2 is also premolariform but its cusps are more robust, with the metacone much larger than on the P1. The metastyle is still low but it is connected to a cingulum that rings the lingual base of the metacone before fading. The B cusp is slightly better defined than on the P1 but still minute, and while a faint cuspule is visible at the mesiobuccal corner of the crown no buccal cingulum is present. The distal root is slightly larger than the mesial root on the P1 and P2. The morphology of the P3 is substantially different from the mesial premolars. The paracone is taller and heavier, with a strong distal crest meeting a large but still much lower metacone. The bases of these two cusps are poorly separated. Cusp B is by far the lowest of the three main cusps and is somewhat mesially inclined, though this cusp is proportionately larger than in the P2 and its base more elevated and detached from the cingulum. The P3 metastyle is much larger than on the P2 but it is wedged against the base of the P4. This relationship is present in both specimens, suggesting that this represents the fully erupted position of the tooth ( Figs. 1B , 2B ). A distinct cingulum is developed mesially and distally on the P3, but it is incomplete at the mid-point of the crown both buccally and lingually at the base of the paracone. Small cuspules are present along the cingula. The paracone and metacone have apical damage but also show evidence of wear; there is a narrow facet developed on the mesiolingual face of the paracone, and the metacone is beveled in an orientation that suggests contact with the paraconid of the lower p4 (a small patch of wear is also present along the distal lingual cingulum on the P3 of the holotype, though this facet is absent from the referred specimen; see discussion of wear and occlusion below). The P4 continues the trend of increasing size and molarization of the premolars. The tooth and its major cusps are arranged along the main axis of the tooth row. The B cusp and metacone are larger than on the P3, but the apices of the three main cusps are close to each other and their bases remain poorly separated. The large, round paracone base occupies most of the crown width. The metastyle is large and connected to the metacone by a sharp crest; the distal face of this cusp has been removed by wear. The mesial-most portion of the crown is too worn to determine if an additional cusp was present (this cusp would be homologous with the stylocone) but, judging from the size of this cusp on the molars where it is preserved, it would likely have been tiny. A complete buccal cingulum is present and bears several small cuspules, the largest of which are at the mesial and distal ends where the cingulum is the broadest. The lingual cingulum is nearly complete except for a small gap lingual to the apex of the paracone (we assume the presence of this cingulum mesially, as it is present on the P3); two or three heavy cuspules were likely present adjacent to the base of the paracone, but portions are heavily worn. While apical wear of the P4 paracone is similar to that on the P3, there is considerably more occlusal wear on the mesiolingual and distolingual flanks of the P4 crown. A large facet is present on the mesial slope of the paracone lingual to the main crest, and a roughly coplanar facet occupies the mesiolingual corner of the crown base. The B cusp has been abraded obliquely, and the cingulum has been obliterated here. Distally, much of the metacone has been removed by wear leaving an oblique, lingually-oriented surface and significant wear is present along the distolingual cingulum (though the cingulum is still distinct). The P5 is very similar to the P 4 in overall proportions, differing in aspects of the cingula and in wear pattern (see discussion below). The buccal cingulum is comparatively wider at the level of the paracone base than on the P4, and the lingual cingulum is complete. A large lingual cingular cusp is present centrally, even with the apex of the paracone. The cingulum mesial to this has been obliterated by wear, and a second large, worn cingular cusp is present distally. Cusp B has been almost completely removed by a deeply incised groove cut into the crown by the p5 protoconid, and much of the metastyle and part of the distal lingual cingulum have been flattened by a distolingually-oriented wear facet ( Figs. 1B , 5 ).

The molars are sharply imbricated relative to the premolars, with the line of their cusps rotated distolingually ( Figs. 1B , 2B , 5 ). Both molars also bear a very low crest bridging the roots. While heavily worn, the relative proportions of the three main cusps on the molars appear to have been different from the premolars—the paracone was still likely the tallest cusp, but the bases of the three cusps are much better separated and have a similar buccal extent, giving the crown a roughly rectangular footprint rather than the elliptical shape of the distal premolars. It also seems likely that the paracone of the P4 and P5 was taller than that of the molars, though wear and breakage leave some uncertainty. As preserved, the M1 is the mesiodistally longest tooth of the series (it is unclear how much of the distal crown has been removed by wear). Judging by their bases cusp B was subequal to the metacone, but substantial wear makes cusp heights impossible to determine with certainty. Cusp B is heavily excavated by wear, similar to P5 ( Fig. 5 ). A small but distinct stylocone is present at the mesial end of the crown, in line with the main cusps and connected to the base of cusp B by a faint crest. The stylocone anchors what remains of the lingual cingulum, which is present only at the mesiolingual corner where the M1 and P5 are in contact. The remainder of the lingual cingulum, along with at least the lingual halves of the paracone and metacone, have been obliterated by wear. The buccal cingulum is complete, well developed, and cuspidate along most of its length. A large cuspule is present at a level between the bases of cusp B and the paracone. The distal end of the crown has been flattened by wear in a plane perpendicular to the line of the main cusps; this margin appears to stop at a crest connected to the metacone and coursing to the distobuccal corner of the crown. The orientation of this crest suggests that the metastyle may have been positioned at this corner instead of in-line with the main cusps, as it is on the premolars. A similar morphology of this crest is present on the M2, supporting deviation of the position of the metastyle. The M2 is a much smaller tooth, about 60% the length of the M1. It is more compact in overall shape, but this is perhaps due to differences in relative wear pattern; both molars show a similar amount of wear indicating that the specimen is a senescent individual. Cusp B is proportionally smaller on M2 than on M1, the buccal cingulum lacks a dominant cuspule, and is slightly narrower distally, but the molars are otherwise generally similar. There is some evidence of wear on the distal face of the M 2 in the holotype specimen suggesting the presence of an additional lower molar (an m3), but this could instead represent damage (a facet does not appear to be present on OMNH 69352).

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 1. 3D surface renderings from CT data of the morganucodontan mammaliaform Cifellilestes ciscoensis gen. et sp. nov. (OMNH 80538, holotype), from the Cisco Mammal Quarry, Utah, USA, Upper Jurassic Morrison Formation. A. Right skull fragment in lateral (A1), medial (A2), ventral (A3), and dorsal (A4) views. B. Dentition only in lingual (B1), occlusal (B2), and buccal (B3) views.

Fig. 2. 3D surface renderings from CT data of the morganucodontan mammaliaform Cifellilestes ciscoensis gen. et sp. nov. (OMNH 69352), from the Cisco Mammal Quarry, Utah, USA, Upper Jurassic Morrison Formation. A. Left skull fragment in lateral (A1), medial (A2), ventral (A3), and dorsal (A4) views. B. Dentition only in lingual (B1), occlusal (B2), and buccal (B3) views.

Fig. 3. Hypothesized life position of the skull fragment (OMNH 80538) of the morganucodontan mammaliaform Cifellilestes ciscoensis gen. et sp. nov., in lateral (A1) and ventral (A2) views. Remainder of the skull based on the composite restoration of Morganucodon by Kermack et al. (1981). Artwork by Hannah Caisse.

Fig. 4. Illustration of the morganucodontan mammaliaform Cifellilestes ciscoensis gen. et sp. nov. (OMNH 80538, holotype); right skull fragment in lateral (A1), medial (A2), ventral (A3), and dorsal (A4) views.

Fig. 5. Illustration of crown morphology of the morganucodontan mammaliaform Cifellilestes ciscoensis gen. et sp. nov. (OMNH 80538 (holotype); right P5–M1 in occlusal view. Terms in parentheses label homologs with traditional “triconodont” cusp nomenclature (e.g., Crompton and Jenkins 1968; Rougier et al. 2007a).