19-8 Cranial Morphology of the First Arvinachelys Specimen in Northern Laramidia

Session: The Campanian Crucible: A synthesis of vertebrate paleobiogeography and ecosystem dynamics in Laramidia

Presenting Author:

Heather Smith

Authors:

Smith, Heather F¹, Berg, Madison², Adrian, Brent³, Schwartz, Gary T.⁴, Fowler, Denver W.⁵, Freedman, Elizabeth A.⁶, Wilson, John P.⁷, Neff, Deanna⁸

(1) Department of Anatomy, Midwestern University, Phoenix, AZ, USA; School of Human Evolution and Social Change, Arizona State University, Tempe, AZ, USA, (2) Department of Anatomy, Midwestern University, Glendale, AZ, USA, (3) School of Human Evolution and Social Change, Arizona State University, PHOENIX, AZ, USA; Institute of Human Origins, Arizona State University, Tempe, AZ, USA, (4) School of Human Evolution and Social Change, Arizona State University, Tempe, AZ, USA; Institute of Human Origins, Arizona State University, Tempe, AZ, USA, (5) Badlands Dinosaur Museum, Dickinson Museum Center, Dickinson, ND, USA, (6) School of Applied Sciences, Dickinson State University, Dickinson, ND, USA; Badlands Dinosaur Museum, Dickinson Museum Center, Dickinson, ND, USA, (7) Department of Geosciences, Princeton University, Princeton, NJ, USA, (8) Badlands Dinosaur Museum, Dickinson Museum Center, Dickinson, ND, USA,

Abstract:

The turtle assemblage of the Campanian Judith River Formation (JRF) is dominated by several baenid species and a diverse array of Pan-Trionychians. Additional vertebrate taxa include chondrichthyan and osteichthyan fishes, squamates, champsosaurs, crocodyliforms, pterosaurs, dinosaurs, and mammals. A partial baenid cranium was recently discovered in the JRF of northcentral Montana, from a region likely equivalent to the lowermost part of the middle Dinosaur Park Formation.

Badlands Dinosaur Museum (BDM) 176.254 is a well-preserved partial turtle skull including the posterior cranial vault, cranial base, and otic capsules, but missing much of the ventral face. It is diagnosed as a baenid by its robust subtriangular form, partial co-ossification of cranial elements, basioccipital-pterygoid contact, and foramen posterior caroticus canalis internus positioned approximately halfway along the pterygoid-basisphenoid suture. It has extensive contact between the pterygoids, lack of parieto-squamosal contact, and parietals larger than frontals with a parieto-frontal suture that is roughly transverse. Nasal bones are prominent, and the external nares are separated at least dorsally by a bony septum, two defining features of Arvinachelys goldeni. However, whether the nares were also fully separated ventrally by the premaxilla, as in A. goldeni, cannot be determined due to the loss of premaxillae in this specimen.

              Arvinachelys is otherwise documented only from the penecontemporaneous Kaiparowits Formation and underlying Wahweap Formation of Utah. While the holotype was CT-scanned and segmented in 2015, advances in imaging software and analyses in the past ten years facilitate more detailed exploration of the internal morphology and intraspecific variation. BDM 176.254 clarifies that the orbits of Arvinachelys are large and laterally oriented, and the frontals are excluded from the anterior orbital margins, but the parietals contribute to their posterodorsal margin. The exoccipitals do not contact each other at the midline, and the condylus occipitalis is composed entirely of basioccipital. Unlike the single hypoglossal foramen in the type specimen, BDM 176.254 has two subequal hypoglossal foramina. The opisthotic contacts the posterior margin of the foramen stapediotemporale, whereas it is excluded in the holotype. Semicircular canals of BDM 176.254 are morphologically similar to those of other baenids. This study extends the geographic distribution of Arvinachelys north into the JRF, clarifies previously unrecognized morphological details, and improves understanding of its cranial variation.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

doi: 10.1130/abs/2026RM-14647

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