New specimens of the early Permian apex predator Varanops brevirostris at Richards Spur, Oklahoma, with histological information about its growth pattern

Abstract

An articulated pelvic region and additional isolated material of Varanops brevirostris, which are indistinguishable from those of the generotype from the Cacops bonebed, demonstrate the presence of this large varanopid at the Richards Spur locality. The articulated specimen includes lumbar, sacral, and anterior caudal vertebrae, partial pelvis, femur, and proximal part of tibia, confirming the autapomorphies previously suggested for this species. These include the presence of distinct blade-like shapes of the neural spines in the sacral region, the presence of deeply excavated pubis, and the presence of a distinct transverse ridge on the ventral surface of the femur distal to the intertrochanteric fossa. It has also been found that the transverse ridges and grooves become larger during ontogeny since the juvenile specimen did not exhibit a well-developed ridge. Histological analysis of isolated limb bones and neutron computed tomography (nCT) of the articulated specimen indicate that the latter likely belonged to an adult individual. This is in contrast to the other varanopid at Richards Spur, the significantly smaller, more gracile predator Mesenosaurus efremovi, which also shows the presence of growth lines and the external fundamental system with an estimated minimum age of fourteen.

Size and shape heterodonty in the early Permian synapsid Mesenosaurus efremovi

“…Therefore, the oldest appearance of true ziphodonty is found in M. efremovi and D. grandis, two carnivorous taxa which coexisted together. Based on its size, the large varanodontine varanopid at Richards Spur, Varanops brevirostris, may have likely been one of the apex predators in the ecosystem (Maddin et al, 2006;Maho et al, 2023); however, its dentition lacked the notable special-…”

“…Therefore, the oldest appearance of true ziphodonty is found in M. efremovi and D. grandis , two carnivorous taxa which coexisted together. Based on its size, the large varanodontine varanopid at Richards Spur, Varanops brevirostris , may have likely been one of the apex predators in the ecosystem (Maddin et al., 2006 ; Maho et al., 2023 ); however, its dentition lacked the notable specializations associated with a hypercarnivorous diet, including serrated carinae. The possibility of a co‐occurring varanopid species and other large predators at Richards Spur implies that changes in the tooth morphology and dental specializations in M. efremovi may have been driven by competition.…”