The Base of the Archosauria and the Origin of the Dinosauria

Updated August 20, 2015 with the addition of figure 1.

Everyone knows that crocs and birds are today’s living Archosaurs. Prehistoric crocs and dinosaurs were closer to the origin of this clade, the subject of today’s blog.

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 1. 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.

Traditionally (as in Nesbitt 2011), at the base of, or just prior to the Archosauria, you’re supposed to find pterosaurs, lagosuchids, chanaresuchids and parasuchians. Gaaak! Unfortunately such results arise from too small of an inclusion set in which unrelated taxa nest by default. We talked about this earlier.

Here (Fig. 1), at the very base of the Archosauria, you’ll find Gracilisuchus (among the crocs), Herrerasaurus (among the dinos) and Trialestes and Turfanosuchus representing more primitive sisters to both.

Turfanosuchus (Young 1973, Fig. 2) had a full set of unreduced fingers and toes, which is good for a basal taxon. Nearly all other archosaurs diminished digits 1 and 5. However, Turfanosuchus represents an herbivorous offshoot because it had an elongated torso, which Gracilisuchus and Herrerasaurus lacked.

Unfortunately, Trialestes (Rieg 1963, Fig. 2) is known from incomplete remains — some of which have only been described in the literature (Clark et al. 2000). Even so, what is known is enough to be important and revealing. I think of it as a cross between Pseudhesperosuchus and Herrerasaurus.

Gracilisuchus (Romer 1972, Fig. 2) was at the base of the crocodylomorpha, but in its own subclade along with Saltopus and Scleromochlus, two expert bipeds. Gracilisuchus was more primitive, just experimenting with bipedality on shorter hind legs and longer front ones.

In anyone’s book, Herrerasaurus (Reig 1963, Fig. 2) is the most primitive dinosaur we know. Even so, while much larger than and quite distinct from Gracilisuchus, the basal dino shared few traits with the basal croc. It was already deep into being a dinosaur with few transitional traits (but see below!). More basal archosaurs, as they are discovered, will look more and more like one of these two or a blend of both.

What Sets Archosaurs Apart?
Here’s a short list:

Basal Archosaurs

Figure 2. Basal archosaurs. Click to enlarge.

At this stage bird and croc ancestors began experimenting with bipedality. Rising up on their hind limbs and furthermore elevating their heels, basal archosaurs became digitigrade bipeds. This freed up their hands to do other things, like flap or become vestiges. An early protoarchosaur, Turfanosuchus, went back to a quadrupedal stance, because it had gone off to the other side and become an herbivore with a big gut that found green immobile food growing at its feet. The meat-eaters of this clade stayed bipedal. Those who chose not to fight for their food did not.

Skull Width
Here’s where basal dinos developed a skull that was narrower than tall and basal crocs developed a skull that was twice as wide as tall. The descending process of the squamosal and the entire quadratojugal tended to become more gracile in dinosaurs.

Temple Change
With a wider skull basal crocs developed their distinctive temple regions with the forward leaning quadrate and quadratojugal contacting or nearly contacting the postorbital. Perhaps this sort of architecture strengthened the temple region on a wider skull.

With a narrower skull, dinosaurs developed a more gracile and more vertical squamosal and quadrate morphology.

Note the nasal in these taxa. It wrapped laterally around the naris unlike most other reptiles.

Postfrontal Fusion
The postfrontal were fused in most Archosaurs, but in different ways. In crocs the postorbital and postfrontal fused. In dinos the frontal and postfrontal fused. Gracilisuchus did not fuse the postorbital and postfrontal. This situation is unclear in its sister, Scleromochlus.

Orbit and Frontal
The orbit is larger than the lateral temporal fenestra in this clade. At least a tip of the frontal enters the upper temporal fenestra in archosaurs.

In basal members of the Archosauria there is no interpterygoid vacuity. Both pterygoids meet or virtually meet each other in a straight line at the midline.

In Gracilisuchus, at the base of the Archosauria, the ribs were expanded (costal plates) which seems to happen whenever tetrapods change locomotion modes, as in Ichthyostega and Thrinaxodon.  Hesperosuchus retained that rib morphology, but closer sisters, like Scleromochlus,  Terrestrisuchus and Herrerasaurus did not.

Radiale and Ulnare
Trialestes had an elongated radiale and ulnare, which is trait found only in crocs back to Pseudhesperosuchus. This may mean the radiale and ulnare were secondarily reduced in Herrerasaurus.

Ankles and the Calcaneal Tuber
In basal archosaurs there isn’t much of a calcaneal tuber. In most dinosaurs it completely disappears, but in poposaurids it enlarges (which has led to phylogenetic confusion). Likewise, in crocodylomorphs the calcaneal tuber (heel) enlarges.

It appears likely that the archosaur precursor, Pseudhesperosuchus, was a digitigrade biped based on metatarsal/toe ratios and limb length ratios. That begat Gracilisuchus and a series of bipedal crocs which were functionally quadradactyl.  Turfanosuchus, which put five digitigrade toes on the ground but reduced the length of the metatarsus. Digit 1 in Trialestes retained an elongated metatarsus, but reduced metatarsal 1 in length and diameter. That set the pattern of Herresaurus and Pampadromaeus, both of which had a similar foot. These patterns set the stage for both the functionally tridactyl feet of theropods and ornithopods, but also set the stage for the functionally quadradactyl feet of poposaurids and the functionally pentadactyl feet of sauropods (Fig. 2), achieved by reducing the long metatarsals (2-4) to the sizes of the smaller and vestigial ones (1 and 5).

Sauropod pedal evolution.

Figure 3. Basal archosaur pedal evolution. Here metatarsal 1 was shorter and more gracile in basal taxa, including theropods and ornithopods, but in basal sauropodomorphs, metatarsal 1 became more robust. In sauropods all the metatarsals became reduced, especially 2-4, and the phalanges of digit 4 were reduced and lost. This demonstrates how a functionally tridactyl pes, as in Herrerasaurus, can become a functionally pentadactyl pes by losing and shrinking elements!!!

Nomenclature Problems
The shifting of taxa here from the traditional tree(s) (Nesbitt 2011) brings with it confirmation and falsification of certain clade names. These we’ll talk about next in a series of four upcoming blogs. (Hint: Say good-bye to “Ornithodira”, “Dinosauriformes” and “Dinosauromorpha”).

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

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Sues H-D, Nesbitt SJ, Berman DS and Henrici AC 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society Bpublished online 
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