A late-surviving phytosaur from the northern Atlantic rift reveals climate constraints on Triassic reptile biogeography

Abstract

Background: The origins of all major living reptile clades, including the one leading to birds, lie in the Triassic. Following the largest mass extinction in Earth's history at the end of the Permian, the earliest definite members of the three major living reptile clades, the turtles (Testudines), crocodylians and birds (Archosauria), and lizards, snakes, amphisbaenians, and Tuatara (Lepidosauria) appeared. Recent analyses of the Triassic reptile fossil record suggest that the earliest diversifications in all three of these clades were tightly controlled by abrupt paleoclimate fluctuations and concordant environmental changes. Yet, this has only been preliminarily tested using information from evolutionary trees. Phytosauria consists of superficially crocodylian-like archosaurs that either form the sister to the crown or are the earliest divergence on the crocodylian stem and are present throughout the Triassic, making this clade an excellent test case for examining this biogeographic hypothesis.

Results: Here, I describe a new phytosaur, Jupijkam paleofluvialis gen. et sp. nov., from the Late Triassic of Nova Scotia, Canada, which at that time sat in northern Pangaea near the northern terminus of the great central Pangean rift. As one of the northernmost occurrences of Phytosauria, J. paleofluvialis provides critical new biogeographic data that enables revised estimations of phytosaur historical biogeography along phylogenies of this clade built under multiple methodologies. Reconstructions of phytosaur historical biogeography based on different phylogenies and biogeographic models suggest that phytosaurs originated in northern Pangaea, spread southward, and then dispersed back northward at least once more during the Late Triassic.

Conclusions: The results presented in this study link phytosaur biogeography to major changes to Triassic global climate and aridity. Together with the earliest dinosaurs and several other reptile lineages, phytosaur diversification and migration appear to have been restricted by the formation and loss of arid belts across the Pangean supercontinent.

Keywords: Biogeography; Climate change; Pangaea; Phylogenetics; Phytosauria; Triassic.

Fig. 1

Locality and horizon of the new phytosaur. Stratigraphic column (a) of Triassic units in the Bay of Fundy region of Nova Scotia, showing the horizon from which the holotype of Jupijkam paleofluvialis was recovered, after Sues and Olsen [90]. Location (b) of the site of discovery of the holotype placed on a global paleogeographic reconstruction by C. Scotese in GPlates

Fig. 2

The partial skull of Jupijkam paleofluvialis gen. et sp. nov. Preorbital skull YPM VPPU 7920 in (a) right lateral, (b) left lateral, and (c) dorsal views

Fig. 3

The anterior rostrum of Jupijkam paleofluvialis gen. et sp. nov. Anterior rostrum portion of YPM VPPU 7920 in (a) left lateral, (b) right lateral, (c) dorsal, and (d) ventral views

Fig. 4

The posterior rostrum of Jupijkam paleofluvialis gen. et sp. nov. Posterior rostrum portion of YPM VPPU 7920 in (a) left lateral, (b) right lateral, (c) dorsal, and (d) ventral views

Fig. 5

Additional cranial anatomy of Jupijkam paleofluvialis gen. et sp. nov. Anterior end of rostrum of YPM VPPU 7920 in (a) right lateral and (b) right lateral-ventral oblique views. Prenarial region of YPM VPPU 7920 in (c) left lateral and (d) left lateral-dorsal oblique views

Fig. 6

Osteoderm of Jupijkam paleofluvialis gen. et sp. nov. Osteoderm of YPM VPPU 7920 in (a) dorsal, (b) ventral, and (c) lateral views

Fig. 7

Comparative anatomy of Jupijkam paleofluvialis gen. et sp. nov. Line drawings of (a) the holotype skull of Jupijkam paleofluvialis YPM VPPU 7920, (b) the skull of Rutiodon carolinensis USNM VP5373 after Colbert [17], and (c) the holotype skull of Machaeroprosopus lottorum TTU-P10076 after Hungerbühler et al. [41], all in left lateral view. The groove in (a) has been reconstructed based on the right side

Fig. 8

Phylogenetic position of Jupijkam paleofluvialis gen. et sp. nov. Strict consensus topology (a) and portions of the two most parsimonious trees recovered (b-c) from the parsimony analysis of the modified dataset of [45]. Numbers at nodes are standard bootstrap supports

Fig. 9

Phylogenetic position of Jupijkam paleofluvialis gen. et sp. nov. continued. Strict consensus topology (a) and portions of two most parsimonious trees recovered (b-c) from the parsimony analysis of the modified dataset of Datta and Ray [22]. Numbers at nodes are standard bootstrap supports

Fig. 10

Bayesian tip-dated phylogeny of phytosaurs and biogeographic reconstruction. Maximum clade credibility tree (a) resulting from the tip-dated Bayesian analysis of the modified dataset of [45] (2018). Bars at nodes indicate 95% highest posterior density (HPD) intervals for node divergence times, numbers at nodes are posterior support values, and colored boxes represent the ancestral areas estimated using BioGeoBears in R. The panel in (b) shows the position of localities sampled in the tree on a simplified map of Pangaea by the United States Geological Survey

Fig. 11

Bayesian tip-dated phylogeny of phytosaurs and biogeographic reconstruction continued. Maximum clade credibility tree (a) resulting from the tip-dated Bayesian analysis of the modified dataset of Datta and Ray [22]. Bars at nodes indicate 95% highest posterior density (HPD) intervals for node divergence times, numbers at nodes are posterior support values, and colored boxes represent the ancestral areas estimated using BioGeoBears in R. The panel in (b) shows the position of localities sampled in the tree on a simplified map of Pangaea by the United States Geological Survey