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Integration of Molecules and New Fossils Supports a Triassic Origin for Lepidosauria (Lizards, Snakes, and Tuatara)

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Integration of Molecules and New Fossils Supports a Triassic Origin for Lepidosauria (Lizards, Snakes, and Tuatara)

Marc E H Jones et al. BMC Evol Biol.

Abstract

Background: Lepidosauria (lizards, snakes, tuatara) is a globally distributed and ecologically important group of over 9,000 reptile species. The earliest fossil records are currently restricted to the Late Triassic and often dated to 227 million years ago (Mya). As these early records include taxa that are relatively derived in their morphology (e.g. Brachyrhinodon), an earlier unknown history of Lepidosauria is implied. However, molecular age estimates for Lepidosauria have been problematic; dates for the most recent common ancestor of all lepidosaurs range between approximately 226 and 289 Mya whereas estimates for crown-group Squamata (lizards and snakes) vary more dramatically: 179 to 294 Mya. This uncertainty restricts inferences regarding the patterns of diversification and evolution of Lepidosauria as a whole.

Results: Here we report on a rhynchocephalian fossil from the Middle Triassic of Germany (Vellberg) that represents the oldest known record of a lepidosaur from anywhere in the world. Reliably dated to 238-240 Mya, this material is about 12 million years older than previously known lepidosaur records and is older than some but not all molecular clock estimates for the origin of lepidosaurs. Using RAG1 sequence data from 76 extant taxa and the new fossil specimens two of several calibrations, we estimate that the most recent common ancestor of Lepidosauria lived at least 242 Mya (238-249.5), and crown-group Squamata originated around 193 Mya (176-213).

Conclusion: A Early/Middle Triassic date for the origin of Lepidosauria disagrees with previous estimates deep within the Permian and suggests the group evolved as part of the faunal recovery after the end-Permain mass extinction as the climate became more humid. Our origin time for crown-group Squamata coincides with shifts towards warmer climates and dramatic changes in fauna and flora. Most major subclades within Squamata originated in the Cretaceous postdating major continental fragmentation. The Vellberg fossil locality is expected to become an important resource for providing a more balanced picture of the Triassic and for bridging gaps in the fossil record of several other major vertebrate groups.

Figures

Figure 1
Figure 1
Geographic and stratigraphic data for the Vellberg jaws. The lepidosaur bearing horizon in the Lower Keuper is marked with a star.
Figure 2
Figure 2
Partial rhynchocephalian dentaries from the Vellberg locality of Germany. A. Dentary SMS 91060. From top to bottom: SEM of labial aspect, drawing of labial aspect, CT model in labial, dorsal and lingual view. B. Dentary SMS 91061. From top to bottom: photo of lingual aspect, drawing of lingual aspect, CT model in lingual, dorsal and labial view. Scale bars equal 1 mm.
Figure 3
Figure 3
Phylogenetic relationships of the fossil jaws based on morphological data from living and extinct taxa. 50% majority rule consensus tree inferred by MrBayes 3.1. Numbers below lines denote posterior probabilities. Numbers above lines denote bootstrap support values obtained from 1000 bootstrap replicates using parsimony criterion in PAUP*. Dashed lines indicate branches found by MrBayes but collapsed in the parsimony analysis, i.e. have less than 50% bootstrap support.
Figure 4
Figure 4
Maximum clade credibility tree (BEAST) with constrained nodes labelled according to Table 2 . Tectonic maps were redrawn from Blakey [58]. CPE indicates the Carnian Pluvial Event [61]. Calibrated nodes are numbered X and 1–12 as in Table 1 but Y, crown Archosauria, is not shown. For results from the MrBayes analysis, including posterior probabilities of separate nodes, see Additional file 5.
Figure 5
Figure 5
The phylogenetic relationships and fossil record of early lepidosaurs compared to molecular divergence estimates. Estimates for the origin of Lepidosauria based on previous molecular studies are listed on the right in blue with short arrows. Estimates for the origin of crown group Squamata are listed on the right in red with long arrows. Timescale based on Gradstein et al. [47]. Fossil records include those described, or referred to, in Butler et al. [105], Carroll [27], Clark and Hernandez [31], Colbert [30], Evans [8,9,26,33,91], Evans and Białynicka [34], Evans and Jones [5], Evans et al., [18], Fraser [22,23,136], Fraser and Benton [11], Heckert et al. [24], Nesbitt [180], Renesto [137], Reynoso [19,150], Robinson [29], Sues and Hopson [13], Sues and Olsen [12], Whiteside [15], and others listed in Evans et al. [181] and Jones et al. [10].

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