Abstract
BackgroundArchosauromorpha originated in the middle–late Permian, radiated during the Triassic, and gave rise to the crown group Archosauria, a highly successful clade of reptiles in terrestrial ecosystems over the last 250 million years. However, scientific attention has mainly focused on the diversification of archosaurs, while their stem lineage (i.e. non-archosaurian archosauromorphs) has often been overlooked in discussions of the evolutionary success of Archosauria. Here, we analyse the cranial disparity of late Permian to Early Jurassic archosauromorphs and make comparisons between non-archosaurian archosauromorphs and archosaurs (including Pseudosuchia and Ornithodira) on the basis of two-dimensional geometric morphometrics.ResultsOur analysis recovers previously unappreciated high morphological disparity for non-archosaurian archosauromorphs, especially during the Middle Triassic, which abruptly declined during the early Late Triassic (Carnian). By contrast, cranial disparity of archosaurs increased from the Middle Triassic into the Late Triassic, declined during the end-Triassic extinction, but re-expanded towards the end of the Early Jurassic.ConclusionsOur study indicates that non-archosaurian archosauromorphs were highly diverse components of terrestrial ecosystems prior to the major radiation of archosaurs, including dinosaurs, while disparity patterns of the Ladinian and Carnian indicate a gradual faunal replacement of stem archosaurs by the crown group, including a short interval of partial overlap in morphospace during the Ladinian.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0761-6) contains supplementary material, which is available to authorized users.
Highlights
Archosauromorpha originated in the middle–late Permian, radiated during the Triassic, and gave rise to the crown group Archosauria, a highly successful clade of reptiles in terrestrial ecosystems over the last 250 million years
Disparity declines strongly in the Hettangian, and subsequently increases again until the Toarcian, with a Morphological disparity of non-archosaurian Archosauromorpha and Archosauria When phytosaurs are treated as members of crown Archosauria (Ezcurra 2016) [36], the cranial disparity of nonarchosaurian archosauromorphs increases from the late Permian to the Ladinian, with significant differences from the Early Triassic to the Anisian (Fig. 3b)
After reaching a maximum in the Ladinian, non-archosaurian archosauromorph cranial disparity decreases in the Carnian, with the last non-archosaurian archosauromorphs occurring in the Rhaetian [46]
Summary
Archosauromorpha originated in the middle–late Permian, radiated during the Triassic, and gave rise to the crown group Archosauria, a highly successful clade of reptiles in terrestrial ecosystems over the last 250 million years. Scientific attention has mainly focused on the diversification of archosaurs, while their stem lineage (i.e. non-archosaurian archosauromorphs) has often been overlooked in discussions of the evolutionary success of Archosauria. Previous work attempting to quantify the morphological diversification of archosauromorphs during the Triassic has focused solely on crown archosaurs [4, 21, 22], and has primarily used discrete characters derived from cladistic data matrices assembled for phylogenetic analyses [4, 22]. The utility of such datasets for quantifying ecological variation is, debated [23]. We compare morphospace occupation and temporal changes in disparity in nonarchosaurian archosauromorphs versus archosaurs, and provide new insights into this important diversification event
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