Abstract
We described newly discovered baenid specimens from the Uintan North American Land Mammal Age (NALMA), in the Uinta Formation, Uinta Basin, Utah. These specimens include a partial skull and several previously undescribed postcranial elements of Baena arenosa, and numerous well-preserved shells of B. arenosa and Chisternon undatum. Baenids from the Uintan NALMA (46.5–40 Ma) are critical in that they provide valuable insight into the morphology and evolution of the diverse and speciose baenid family near the end of its extensive radiation, just prior to the disappearance of this clade from the fossil record. These Uintan specimens greatly increase the known variation in these late-surviving taxa and indicate that several characters thought to define these species should be reassessed. The partial cranium of B. arenosa, including portions of the basicranium, neurocranium, face, and lower jaw, was recently recovered from Uinta B sediments. While its morphology is consistent with known specimens of B. arenosa, we observed several distinct differences: a crescent-shaped condylus occipitalis that is concave dorsally, tuberculum basioccipitale that flare out laterally, and a distinct frontal-nasal suture. The current sample of plastral and carapacial morphology considerably expands the documented variation in the hypodigms of B. arenosa and C. undatum. Novel shell characters observed include sigmoidal extragular-humeral sulci, and small, subtriangular gular scutes. Subadult specimens reveal ontogenetic processes in both taxa, and demonstrate that diagnostic morphological differences between them were present from an early developmental age.
Highlights
The Baenidae [1] were an extinct clade of North American freshwater river turtles with an extensive radiation spanning from the early Cretaceous to the middle Eocene [2,3,4,5]
The baenid clade disappeared entirely from the fossil record during the Eocene, with the last documented appearance of this prolific family occurring during the Uintan North American Land Mammal Age (NALMA; 46.5–40 Ma) [8,9,10,11]
[20] Referred specimens: Specimen UMNH Natural History Museum of Utah (VP) 27535 includes a fragmentary skull of Baena arenosa (Fig 2A–2F), as well as numerous associated incomplete shell elements and several limb bone fragments (Fig 3)
Summary
The Baenidae [1] were an extinct clade of North American freshwater river turtles with an extensive radiation spanning from the early Cretaceous to the middle Eocene [2,3,4,5]. Changing climatic conditions and local drainage reorganization including the westward regression of Lake Uinta (a major hydrologic feature of the early Eocene) likely had a profound effect on the evolution of baenids as they are no longer present in the Paleogene record after this epoch [17,18,19]. These late surviving Uintan baenids provide insight into their evolution and ecological adaptations just prior to the extinction of this previously abundant and specious clade
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