The Origin of Dinosaurs – Updated Again

Earlier we looked at the origin of dinosaurs here, here and here.

Today we’ll take the linear approach, documenting the origin of dinosaurs with to scale images (Fig. 1), constant length specimens (Fig. 2) and skull-only specimens (Fig. 3).

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Traditional workers (Irmis et al. 2007, Nesbitt 2011, Brusatte et al. 2010) add pterosaurs and Lagerpeton to the lineage of dinosaurs, but they are not related, according to the large reptile tree (at last updated!).

Derived from a sister to Vjushkovia, Decuriasuchus nests outside the Rauisuchia, but has many traits found in that clade. Notably it retained a simple axle acetabular joint, while rauisuchians overlapped the top of the femur with the ilium.

Figure 2. The origin of dinosaurs using constant length specimens. Here Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes and Herrerasaurus not to scale.

Figure 2. The origin of dinosaurs using constant length specimens. Here Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes and Herrerasaurus not to scale.

Bipedal locomotion
was an early innovation, even if only occasionally in long-necked Turfanosuchus, which gave rise to poposaurs, and wide-skulled Gracilisuchus, which gave rise to basal bipedal crocs.  Miniaturization occurred at the base of the Archosauria and the base of the Dinosauria.

Figure 2. Origin of dinosaurs documented by skulls shown at constant length in evolutionary order. Shown here are Decuriasuchus, Turfanosuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Eoraptor and Tawa, not to scale.

Figure 2. Origin of dinosaurs documented by skulls shown at constant length in evolutionary order. Shown here are Decuriasuchus, Turfanosuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Eoraptor and Tawa, not to scale. Despite the evolutionary progress of the skulls, each one also represents a distinct branch not on the single lineage of dinosaurs, a lineage that will never be found precisely, but only estimated using sister taxa shown here and those yet to be discovered.

Figure 5. Family tree of the Archosauria and basal Dinosauria. Bootstrap scores are shown.

Figure 5. Family tree of the Archosauria and basal Dinosauria. Bootstrap scores are shown.

 

 

Relative to basal crocs,
basal dinos had a taller, narrower skull first seen in tiny Lewisuchus and Pseudhesperosuchus, which further developed a very tall postorbital.

Most basal dinosaurs, like Tawa (a basal theropod) and Eoraptor (a basal phytodinosaur) had a gracile premaxilla, but Herrerasaurus had a robust premaxilla with overlapping nasals to reinforce its rostrum.

Family Tree
The tree at left represents the latest thinking with Lewisuchus at the base of the previously unrecognized proto-dinosaurs. If dinosaurs are defined as birds + Triceratops, then the grade Lewisuchus to Herrerasaurus are not dinos in this tree even though they are  on the same lineage.

I hesitate to use the term Dinosauromorpha because it was defined by Sereno 1991 as: “the last common ancestor of Lagerpeton chanarensisMarasuchus lilloensisPseudolagosuchus major and the Dinosauria (including Aves) and all its descendants.” Unfortunately that definition is redundant with the Archosauriformes as it includes Proterosuchus. You can see where Lagerpeton nests here

References
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wiki/Decuriasuchus
wiki/Lewisuchus
wiki/Gracilisuchus
wiki/Trialestes
wiki/Pseudhesperosuchus
wiki/Eoraptor
wiki/Tawa
wiki/Herrerasaurus
wiki/Sanjuansaurus

 

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