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| Sarabosaurus dahli Polcyn, Bardet, Albright & Titus, 2023 Artwork by Andrey Atuchin |
Abstract
We describe and name a new mosasaur taxon, Sarabosaurus dahli gen. et sp. nov., from the lower Turonian part of the Tropic Shale in Utah, USA. The holotype specimen preserves significant portions of the skull and axial postcranial skeleton. It was found in the upper part of the Watinoceras devonense Ammonite Zone, bounded by radioisotopic dates above and below, and is thus about 93.7 Ma, the oldest mosasaurid taxon known from the Western Interior Seaway. The new taxon possesses a vascular pattern of the basisphenoid heretofore only seen in late diverging plioplatecarpine mosasaurids. Reevaluation of the morphology of the basisphenoid of previously described Turonian mosasaurs using μCT techniques reveals the derived condition is also present in Yaguasaurus and the incipient condition in Tethysaurus and Russellosaurus. In these two taxa, the canals enter the basisphenoid, but do not pass into the basioccipital. Instead, they exit only high on the posterior wall of the sella turcica, in a position similar to the basilar artery of other lizards. This vascular pattern, both in its incipient and derived states, is unique among squamates and supports inclusion of the aforementioned taxa in a monophyletic Plioplatecarpinae, for which we provide an emended diagnosis. Phylogenetic analysis recovers Sarabosaurus dahli gen. et sp. nov. as the sister taxon to Yaguarasaurus and all other later diverging plioplatecarpines, with Russellosaurus and Tethysaurus as successive sister taxa. Tylosaurine mosasaurids retain the primitive condition of the basisphenoid vascularization pattern and implies a tylosaurine-plioplatecarpine divergence in the late Cenomanian or earliest Turonian.
Keywords: Plioplatecarpinae, Tylosaurinae, Turonian, Mosasauridae, Tropic Shale
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| Schematic reconstruction of the skull of Sarabosaurus dahli gen. et sp. nov. showing preserved elements in (A) lateral, (B) dorsal, and (C) ventral views. Some elements mirrored from opposite side. Reconstructed portions based on Tethysaurus nopscai (SMU75486) and Russellosaurus coheni (SMU73056). (D) Isolated zygopophysis showing growth rings with relative topographic relief shown in the third panel with legend given in microns, and (E), broken edge of zygosphene showing laminar bone deposition, interpreted as corresponding to annual growth. Scale bars equal 10 cm for A-C and 1mm for D and E. |
Systematic paleontology
Reptilia Linnaeus, 1758
Squamata Oppel, 1811
Mosasauridae Gervais, 1852
Russellosaurina Polcyn and Bell, 2005
Plioplatecarpinae Dollo, 1884
Included Genera— Platecarpus Cope, 1869; Plioplatecarpus Dollo, 1882; Angolasaurus Antunes, 1964; Ectenosaurus Russell, 1967; Selmasaurus Wright and Shannon, 1988; Yaguarasaurus Paramo, 1994; Tethysaurus Bardet et al., 2003; Russellosaurus Polcyn and Bell, 2005; Latoplatecarpus Konishi and Caldwell, 2011; Plesioplatecarpus Konishi and Caldwell, 2011; Romeosaurus Palci et al., 2013; Gavialimimus Strong et al. 2020.
Emended Diagnosis. (compare with Russell, 1967; Bell, 1997; Lingham-Soliar, 1994; Konishi and Caldwell, 2011); small to medium size mosasaurids (2-6 meters); canals for basilar artery enter basisphenoid below abducens nerve exits and above internal carotid artery path, internally converging medially giving rise to the basilar artery, and in some taxa continue posteriorly within the basisphenoid and basioccipital, exiting on medullary floor of the basioccipital as paired canals or a single bilobate canal; premaxilla bears ventral median ridge just posterior to dentigerous portion; quadrate suprastapedial process elongate, reaching mid-height of quadrate in most taxa; two or three foramina on ventrolateral face of retroarticular process in most taxa; marginal tooth crown with sub-circular basal cross-section and medially finely striated, laterally faceted or fluted to various extent; parietal rami-supratemporal contact obliquely or horizontally oriented.
Remarks. The novel vascular pattern present in the basisphenoid of the Sarabosaurus dahli gen. et sp. nov. described herein is shared with Tethysaurus, Russellosaurus, Yaguarasaurus, and all other mosasaurid genera previously recognized as belonging to the subfamily Plioplatecarpinae (sensu Konishi and Caldwell, 2011). This vascular pattern is unique amongst mosasaurs and non-mosasaurian squamates and strongly supports the monophyly of this more inclusive arrangement to the exclusion of Tylosaurinae contra Konishi and Caldwell (2011), justifying the newly emended Plioplatecarpinae.
Sarabosaurus gen. nov.
Etymology. Sarabosaurus (Suh-raib-o-sawr-us) is derived from ’Sarab’, an Arabic word meaning ‘desert mirage’ and the Greek ‘sauros’ meaning lizard. The ’lizard of the desert mirage’ pays homage to the mirages often seen in the hot Tropic Shale badlands in the summer, which provided but a glimpse of the life that flourished in the long vanished Cretaceous Western Interior Seaway.
Diagnosis. The new taxon is diagnosed by the following unique combination of characters: premaxilla is anteriorly blunt, broadly arcuate in dorsal view with a small edentulous rostrum, ethmoid nerve enters premaxilla in a deep position, well below the dorsal surface and exits on dorsal surface as large irregularly spaced foramina, atavistic or paedomorphic retention of egg tooth, anterior internarial bar triangular in cross section; dermal sculpting present on the lateral face of maxillae; supradental portion of maxilla at external naris is transversely broader than tall; frontal supraorbital region is nearly square with dermal sculpting on the dorsal surface; prefrontal and postorbitofrontal separated by frontal supraorbitally, posterolateral alae of frontal rounded, posterior terminus of squamosal tear-drop-shaped without dorsal (parietal) process; relatively large lumina of semicircular canals; posteromedial vascular canal enters basisphenoid below abducens nerve exit, giving rise to basilar artery anteriorly and posteriorly forming a central vestibule within basisphenoid, pterygoid tooth row not elevated, sigmoidal, and bears at least 14 tooth positions, some of which are present on basisphenoid ramus; quadrate ventral body rostrocaudally thin, expanded ventral medial flange, small posterior ‘infrastapedial’ process, and poorly developed posteroventral ascending alar rim; dentary robust with small prow in front of first tooth position and medial ridge exposes nearly entire tooth root; tall well-developed median ridge on splenial; large zygosphenes present to at least the sacral vertebrae; depressed oval condyles on cervical and anterior trunk vertebrae, condyles on sacral and pygal vertebrae slightly taller than wide; anterior caudal vertebrae wider than tall; mid to posterior caudal vertebrae taller than wide.
Sarabosaurus dahli gen. et sp. nov..
Holotype. UMNH VP21800 (GLCA 24271), fragments of cranium, mandible, and vertebrae representing cervical, dorsal, pygal, and caudal series.
Type Locality. GLCA site 327, Glen Canyon National Recreation Area, Kane County, Utah, USA (detailed locality data on file at UMNH).
Stratigraphic Occurrence. Lower part of the Mytiloides kossmati Inoceramid biozone, upper Pseudaspidoceras flexuosum North American ammonoid biozone.
Age. Early Turonian, bounded by radioisotopic dates on bentonites and constraining the horizon that produced the specimen to about 93.7 Ma.
Etymology. The species epithet, dahli, is in honor of the many contributions of Steve Dahl, longtime volunteer at Grand Staircase-Escalante National Monument, Kanab, Utah.
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| (A) Turonian paleogeography with political boundary overlay showing site GLCA 327. (B) Reference map of study area along southern border of Kaiparowits Plateau. Finely dotted line separates Glen Canyon National Recreation Area from Grand Staircase-Escalante National Monument. Star denotes location of type specimen. Modified from Albright et al. (2007a). (C) Generalized stratigraphic column of GLCA 327. A, B, C, and D, are bentonites of Elder (1991).Abbreviations: LS, Limestone beds. Lim., Limonite bed, and approximate local stratigraphic position of Cenomanian-Turonian boundary. (D) GLCA 327 during recovery of (UMNH VP 21800/GLCA 24271), Sarabosaurus dahli gen. et sp. nov.. Base map in A licensed from Colorado Plateau Geosystems, Inc. |
Conclusions:
Herein we described and named a new mosasaur taxon, Sarabosaurus dahli gen. et sp. nov., from the Watinoceras devonense Ammonite Zone of the Tropic Shale, Utah. The holotype specimen is confidently dated at about 93.7 Ma based on radioisotopic dates from bentonites above and below the horizon from which it was preserved, and is thus the oldest named mosasaur taxon known from the Western Interior Seaway. Although the specimen exhibits numerous plesiomorphic traits, remarkably, the basisphenoid preserved with the specimen shows the derived plioplatecarpine basicranial circulation pattern, which is unique amongst squamates, and represents a key evolutionary novelty. We describe the basicranial circulation pattern in select mosasaurids, identifying a morphocline consisting of three types. We show the mosasaur subfamilies Tylosaurinae, Mosasaurinae, and Halisaurinae retain the plesiomorphic squamate condition (Type1), while de novo vascular morphology supplants the basilar artery in Tethysaurus and Russellosaurus (Type 2), and the advanced plioplatecarpine (Type 3) condition is present in Yaguarasaurus and all other plioplatecarpines in which it can be assessed. Though some details remain unknown due to incomplete preservation of the basioccipital and the basisphenoid, Sarabosaurus dahli gen. et sp. nov. is inferred to possess the Type 3 condition because the basicranial canal for the basilar artery trends posteromedially within the basisphenoid as in later diverging plioplatecarpines. Recognition of the incipient (Type 2) condition in Tethysaurus and Russellosaurus, and the Type 3 condition in Yaguarasaurus and Sarabosaurus dahli gen. et sp. nov., provides the basis for an emended diagnosis of a more inclusive Plioplatecarpinae. The remarkable influence of these new data on the branching patterns recovered in our phylogenetic analysis, underscores the importance of discovery of new informative specimens, and the application of technology such as μCT in the search for new characters that elucidate the basal relationships and evolutionary patterns of mosasaurids.
Michael J. Polcyn, Nathalie Bardet, L. Barry Albright III and Alan Titus. 2023. A New lower Turonian mosasaurid from the Western Interior Seaway and the Antiquity of the unique basicranial circulation pattern in Plioplatecarpinae. Cretaceous Research. 105621; In Press. DOI: 10.1016/j.cretres.2023.105621
