High morphological disparity in a bizarre Paleocene fauna of predatory freshwater reptiles

Abstract

Background: The consequences of the K-Pg mass extinction are reflected across present biodiversity, but many faunas that appeared immediately after the extinction event were very different from current ones. Choristodera is a clade of reptiles of uncertain phylogenetic placement that have an extremely poor fossil record throughout their 150-million-year history. Yet, choristoderes survived the K-Pg event and persisted until the Miocene.

Results: I describe the skulls and skeletons of two new choristoderes from a single Paleocene ecosystem in western North America that reveal the hidden Cenozoic diversity of this reptile clade. Despite their similar size, the new species deviate dramatically in morphology. Kosmodraco magnicornis gen. et sp. nov. possesses an extremely short snout and extensive cranial ornamentation. The sacrum of K. magnicornis bears enlarged muscle attachment sites and other modifications reminiscent of some giant crocodylians. In contrast, Champsosaurus norelli sp. nov. is a longirostrine species with an uninflated and ventrally divergent postorbital skull. Together with a North American choristodere previously classified in the European genus Simoedosaurus, K. magnicornis substantiates a new clade of giant, short-snouted taxa endemic to the Americas. C. norelli is found to be an early-diverging member of the genus Champsosaurus from the Cretaceous-Paleogene of the northern hemisphere. This suggests the presence of several ghost lineages of champsosaurid that crossed the K-Pg boundary.

Conclusions: The new taxa greatly increase Cenozoic choristodere richness and strengthen the evidence for the existence of distinctive freshwater faunas in Paleogene Eurasia and North America, where this clade diversified to exploit newly available macropredatory niches in the aftermath of the asteroid impact. The new choristoderes also reveal the distinct ecological context in which extant freshwater predators of the Americas like alligatoroids and gars have their origins: Paleocene fluviolacustrine ecosystems in North America displayed high large predator diversity and morphological disparity relative to modern ones.

Keywords: Biogeography; Cenozoic; Choristodera; Ecosystem recovery; K-Pg; Phylogenetics.

Fig. 1

Cranial anatomy of Kosmodraco magnicornis gen. et sp. nov. Holotype skull YPM VPPU 19168 in a dorsal, b ventral, c anterior, and d right lateral views, with details of the palatal anatomy at the anterior end e and midway along f the skull. Inset black and white drawings illustrate borders between bones

Fig. 2

Cranial anatomy of Kosmodraco magnicornis gen. et sp. nov. Referred specimen YPM VPPU 18724 in a dorsal, and b ventral views, with complete left c, d and right mandibles e, f in c, e lateral and d, f dorsal views, and digitally articulated left and right dentary symphyses in g ventral view. Inset black and white drawings illustrate borders between bones

Fig. 3

Lateral cranial profile of Kosmodraco magnicornis gen. et sp. nov. and Alligator mississippiensis compared. Skulls of K. magnicornis (a) and A. mississippiensis (b) in right lateral view (not to scale)

Fig. 4

Postcranial anatomy of Kosmodraco magnicornis gen. et sp. nov. Left shoulder girdle of K. magnicornis holotype YPM VPPU 19168 in a lateral and b medial views

Fig. 5

Postcranial anatomy of Kosmodraco magnicornis gen. et sp. nov. Dorsal vertebrae of referred specimen in a, c left lateral and b, d anterior views

Fig. 6

Postcranial anatomy of Kosmodraco magnicornis gen. et sp. nov. Semi-articulated posterior dorsals, sacrum, and anterior caudals of referred specimen in a dorsal, b anterior, and c lateral views, with d, detail of the disarticulated dorsals still embedded in the block. Arrows point to rugosities on sacral ribs for soft tissue attachment

Fig. 7

Postcranial anatomy of Kosmodraco magnicornis gen. et sp. nov. Selected well-preserved caudal vertebrae of referred specimen in lateral view

Fig. 8

Cranial anatomy of Champsosaurus norelli gen. et sp. nov. Skull in dorsal (a), ventral (b), and left lateral (c) views. Braincase in d right lateral, e left lateral, f dorsal, g ventral, and h posterior views. Inset black and white drawings illustrate borders between bones

Fig. 9

Mandibular anatomy of Champsosaurus norelli gen. et sp. nov. Complete right mandible in a lateral, b medial, and c dorsal views. Partial left mandible in d medial, e lateral, and f dorsal views

Fig. 10

Postcranial anatomy of Champsosaurus norelli gen. et sp. nov. Left a and right b scapulae in a lateral and b medial views. Right coracoid in c medial view. Interclavicle in d anterior view. Proximal left humerus in e anterior view. Right humerus in f lateral and g medial views. Right ulna in h lateral and i medial views. Dorsal centrum in j dorsal and k lateral views. Proximal femur fragment in l anterior view. Ribs (m) of C. norelli

Fig. 11

Results of the phylogenetic analysis of choristodere interrelationships under parsimony. a Strict consensus topology from the analysis of the phylogenetic matrix under parsimony. Different most parsimonious subtrees b, c showing conflicting topologies within Neochoristodera, but consistent placement of Tchoiria spp. and Ikechosaurus spp. as early-diverging simoedosaurids and champsosaurids, respectively. Key clades highlighted (Champsosaurus in green, Kosmodraco in blue, and derived "allochoristoderes" in yellow). Champsosaurus silhouette public domain from phylopic.org

Fig. 12

Results of the Bayesian phylogenetic analysis of choristodere interrelationships. Time-calibrated maximum clade credibility tree of choristoderes showing divergence time 95% confidence intervals (blue bars at nodes) and geographic distribution of choristodere species

Fig. 13

Comparison of the palatal anatomy of choristoderes described in this study with those of extant large-bodied freshwater species found in North America. Note the absence of palatal tooth plates in Alligator mississippiensis and the similar placement of the palatal tooth rows in †K. magnicornis and A. spatula. Also note the similarities in alveolar size changes throughout the tooth rows of K. magnicornis and A. mississippiensis. ^†Champsosaurus a n d Alligator silhouettes public domain from phylopic.org.

Fig. 14

Comparison of posterior skull ornamentation in ^†Kosmodraco magnicornis and Alligator mississippiensis. Arrows indicate knob-like ornaments projecting from the squamosal and quadratojugal of ^†K. magnicornis