A new caimanine alligatorid from the Middle Eocene of Southwest Texas and implications for spatial and temporal shifts in Paleogene crocodyliform diversity

Abstract

Dramatic early Cenozoic climatic shifts resulted in faunal reorganization on a global scale. Among vertebrates, multiple groups of mammals (e.g., adapiform and omomyiform primates, mesonychids, taeniodonts, dichobunid artiodactyls) are well known from the Western Interior of North America in the warm, greenhouse conditions of the early Eocene, but a dramatic drop in the diversity of these groups, along with the introduction of more dry-tolerant taxa, occurred near the Eocene–Oligocene boundary. Crocodyliforms underwent a striking loss of diversity at this time as well. Pre-Uintan crocodyliform assemblages in the central Western Interior are characterized by multiple taxa, whereas Chadronian assemblages are depauperate with only Alligator prenasalis previously known. Crocodyliform diversity through the intervening Uintan and Duchesnean is not well understood. The middle Eocene Devil’s Graveyard Formation (DGF) of southwest Texas provides new data from southern latitudes during that crucial period. A new specimen from the middle member of the DGF (late Uintan–Duchesnean) is the most complete cranial material of an alligatorid known from Paleogene deposits outside the Western Interior. We identify this specimen as a caimanine based on notched descending laminae of the pterygoids posterior to the choanae and long descending processes of the exoccipitals that are in contact with the basioccipital tubera. Unlike Eocaiman cavernensis, the anterior palatine process is rounded rather than quadrangular. The relationships and age of this new taxon support the hypothesis that the modern distribution of caimanines represents a contraction of a more expansive early Cenozoic distribution. We hypothesize that the range of caimanines tracked shifting warm, humid climatic conditions that contracted latitudinally toward the hothouse-icehouse transition later in the Eocene.

Phylogenetic analysis of a new morphological dataset elucidates the evolutionary history of Crocodylia and resolves the long-standing gharial problem

“…Grigg et al, 1998;Brochu & Carbot-Chanona, 2015), the crossing of a small marine barrier might be the most likely scenario. The phylogenetic relationships and biogeographic history of the recently erected caimainine Chinatichampsus wilsonorum from the middle Eocene of North America (Stocker, Brochu & Kirk, 2021) remains uncertain.…”

“…This dearth of Oligocene occurrences characterises much of the global record of Crocodylia in general (Markwick, 1998;Mannion et al, 2019;De Celis, Narváez & Ortega, 2020;Solórzano et al, 2020). Although sampling bias almost certainly plays a role in this apparent decline, it also likely reflects genuine latitudinal range retraction during the Eocene-Oligocene transition (Markwick, 1998;Martin, 2010a;Mannion et al, 2015;Whiting & Hastings, 2015;Jouve, Khalloufi & Zouhri, 2019;Macaluso et al, 2019;Stocker, Brochu & Kirk, 2021), which was characterised by global cooling and increased aridity (e.g. Zachos et al, 2001;Zanazzi et al, 2007;Hren et al, 2013;Li et al, 2016).…”

Oldest record of Alligator in southeastern North America

“…Unfortunately, the premaxilla is unknown for this taxon. A newly described taxon from the Eocene of Texas, Chinatichampsus wilsonorum includes a skull with a partial premaxilla (Stocker et al, 2021). This bone preserves an occlusal pit lying medial to the tooth row and nearly entirely anterior to the premaxillary-maxillary suture.…”

“…This is unlike the occlusal pit of UF 422816, which is much more in-line with the tooth row and split across the suture. Premaxillary tooth positions were not assigned for Chinatichampsus, but images indicate the relative proportions of the alveoli anterior to the premaxillary-maxillary suture are in quite different proportions from UF 422816 (Stocker et al, 2021). Moreover, Chinatichampsus also has dorsal exposure of the supraoccipital bone (Stocker et al, 2021).…”

On the homology of crocodylian post‐dentary bones and their macroevolution throughout Pseudosuchia

“…Regarding the phylogenetic position of Thalattosuchia, we consider this clade at the base of Neosuchia (e.g., Clark, 1994;), but we have also discussed results qualitatively considering their alternative placement around the base or as the sister group of Crocodyliformes (e.g., Wilberg, 2015;Young & De Andrade, 2009). The phylogenetic interrelationships among eusuchians follow those of the latest iteration of the phylogenetic analysis of Brochu (1999;Stocker et al, 2021). Finally, we used these references to build an informal archosauriform supertree after merging the different topologies.…”