close The archosaur skull includes two new fenestra (skull openings). The first of these is the mandibular fenestra, an opening through the jawbone. The other is the antorbital fenestra, a term that means "the opening in front of the eye." Archosauria (the "ruling reptiles") is a major group of diapsids, differentiated from the other diapsids by the presence of single openings in each side of the skull, in front of the eyes (antorbital fenestrae), among other characteristics. This continues the tetrapod trend of the reduction of skull bones by the fusion of multiple bones and the opening of fenestrae in the skull. This helps to lighten the skull, provides more room for muscles and other tissues, and allows more skull flexibility (kinesis) when eating. Other typical archosaurian characteristics include another opening in the lower jaw (the mandibular fenestra), a high narrow skull with a pointed snout, teeth set in sockets (called thecodont tooth implantation), and a modified ankle joint.
The fossil recordStudents of archosaur evolution are blessed with a wonderful fossil record for many groups of archosaurs, including some very bizarre extinct taxa. The first archosauromorphs (relatives of the true archosaurs) appear in the fossil record in the Early Triassic, about 245 million years ago, just after the great end-Permian extinction. They include weird hippo-size beaked herbivores called rhynchosaurs, long-necked reptiles called prolacertiforms, evil-looking terrestrial predators like the erythrosuchids and proterosuchids, and close relatives of the true archosaurs, including Euparkeria. Many of these early groups are limited to the Triassic Period, not enduring the extinctions in the Upper Triassic that the dinosaurs and other taxa survived.
Krebs  noted that preparation undertaken by O. Abel between 1936 and 1938 had demonstrated that only a single individual is present, with its vertebral column split in half along a near sagittal plane. The specimen was redescribed and reinterpreted by Krebs , who recognised its archosaurian nature, referred it to Pseudosuchia (currently considered to represent the crocodilian total group), and noted similarities with a then undescribed sail-backed archosaur from the Middle Triassic (late Anisian) Lifua Member of the Manda Beds of Tanzania, more recently named Hypselorhachis mirabilis , . Both Ctenosauriscus and Hypselorhachis were referred to the family Ctenosauriscidae by Krebs .
Our 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.
Our 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.
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, however, debated . Here we attempt for the first time to quantify patterns of cranial morphological diversity during the first 100 million years of the early archosauromorph radiation using an alternative approach that has great power to capture morphological variation - geometric morphometrics. We compare morphospace occupation and temporal changes in disparity in non-archosaurian archosauromorphs versus archosaurs, and provide new insights into this important diversification event.
We used the principal component (PC) scores to calculate temporal and phylogenetic variation in morphological disparity [33, 34]. Temporal disparity curves were calculated for two main groupings of archosauromorphs: the paraphyletic assemblage of non-archosaurian archosauromorphs; and crown group Archosauria. Because recent cladistic analyses find differing placements for phytosaurs, a major group of long-snouted semi-aquatic Triassic archosauromorphs, we constructed two datasets: one in which phytosaurs are included within crown Archosauria [1, 5, 35, 36] and the other in which they are treated as non-archosaurian archosauromorphs . However, our preferred hypothesis is that they are crown archosaurs based upon the most extensive phylogenetic analysis of early archosauromorphs conducted to date . Within crown Archosauria, we also compiled disparity curves for the two main subgroups: Pseudosuchia (crocodile line archosaurs) and Ornithodira (dinosaurs and their close relatives). The measures for pseudosuchians were calculated both including and excluding phytosaurs.
a Temporal variation of disparity for all archosauromorphs through time from the late Permian to the late Early Jurassic. b Temporal pattern of non-archosaurian archosauromorphs (solid line with black squares) and crown archosaurs (dashed line with black diamonds) when phytosaurs are members of the crown. c Temporal pattern of non-archosaurian archosauromorphs (solid line with black squares), pseudosuchians (dashed line with grey circles), and ornithodirans (dashed line with black circles). Significant changes between subsequent time bins are marked with an asterisk. Lines with tiny dots mark missing time bins due to small sample sizes. All silhouettes taken from (www.phylopic.org)
When phytosaurs are treated as members of crown Archosauria (Ezcurra 2016) , the cranial disparity of non-archosaurian 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 (the last bin sampled for the group), with the last non-archosaurian archosauromorphs occurring in the Rhaetian . The first record of Archosauria is found in the Early Triassic  and from this point their cranial disparity increases continuously until the late Norian, followed by a decline in the Hettangian and a significant re-expansion until the Toarcian. The post-Hettangian increase includes a significant change from the Hettangian to the Sinemurian. The cranial disparity of non-archosaurian archosauromorphs is significantly higher than that of archosaurs (including phytosaurs) in the Early and Middle Triassic, whereas archosaurs possess a higher disparity in the Carnian, although the latter is not significant (Fig. 3a, see Additional file 1: Tables S7, S9, S13).
Based on the NPMANOVA, non-archosaurian archosauromorphs exhibit significant shifts of morphospace from the late Permian to the Early Triassic, from the Early Triassic to the Anisian and from the Ladinian to the Carnian, while in Archosauria significant shifts occur from the late Norian to the Hettangian and from the Sinemurian to the Toarcian. When compared to each other, non-archosaurian Archosauromorpha and Archosauria occupy significantly different areas within morphospace during the Early Triassic and the Carnian, but are not significantly separated from each other in the Ladinian (only two Anisian archosaurs are sampled and differences in morphospace with non-archosaurian archosauromorphs cannot be tested statistically) (Fig. 4, see Additional file 1: S9, S13, S16). Results showing the cranial disparity and morphospace occupation when phytosaurs are sister group of crown-archosaurs and when hypothetical ancestors are not included are described in the Additional files (see Additional file 1: Tables S7, S9, S13, S17; S18-S23, Figure S6).
Morphospace occupation of archosauromorphs. a Two-dimensional morphospace of all Early (grey dashed line with white pentagons, silhouette marked with asterisk) and Middle Triassic non-archosaurian archosauromorphs (grey solid line with grey pentagons) and pseudosuchians (black solid line with grey circles). b Two-dimensional morphospace of Late Triassic archosauromorphs showing the morphospace of non-archosaurian archosauromorphs (grey solid line with grey pentagons) and crown archosaurs (black solid line) with pseudosuchians (without phytosaurs) (grey circles), phytosaurs (white squares), and ornithodirans (white circles). c Two-dimensional morphospace of all Early Jurassic archosaurs with ornithodirans (black dashed line with white circles) and pseudosuchians (black solid line with grey circles). Note that graphs show the morphospace of entire epochs and are not always equivalent to the time bins. All silhouettes taken from (www.phylopic.org)
The disparity patterns documented here are consistent with a gradual faunal replacement event, with a short interval of partial morphospace overlap between non-archosaurian archosauromorphs and archosaurs during the Ladinian, followed by decline of the former group and expansion of the morphospace of the latter. The macroevolutionary events described above change if phytosaurs are considered to be outside of Archosauria (see Additional file 1). Nevertheless, even if phytosaurs are not crown archosaurs, the overall evolutionary patterns found here (including the Middle Triassic non-archosaurian archosauromorph disparity peak) are still valid for the group composed of non-archosaurian archosauromorphs to the exclusion of phytosaurs and other archosaurs. 041b061a72