Brusatte et al. (2011) reported, “the earliest phase of dinosaur history remains poorly understood…Here, we report footprints from the Early–Middle Triassic of Poland… which shifts the origin of the dinosaur stem lineage back to the Early Olenekian (ca 249–251 Ma, Fig. 1), approximately 5–9 Myr earlier than indicated by body fossils, earlier than demonstrated by previous footprint records, and just a few million years after the Permian/Triassic mass extinction (252.3Ma).” The two featured ichites included Prorotodactylus and Sphingopus.
Unfortunately, Prorotodactylus ichnites (Fig. 2) did not belong to dinosaur stem taxa. According to the large tree, no dinosaur sisters had digit 4 longer than 3 going back to Proterosuchus and Garjainia. Brusatte et al. (2011) considered Lagerpeton (another taxon with an elongated digit 4) a dinosaur stem taxon, but the large tree nests Lagerpeton with the chanaresuchid Tropidosuchus, unrelated to the Dinosauria. Prorotodactylus ichnites more closely match Macrocnemus sisters (Fig. 3) with pedal digit 4 longer than 3. Brusatte et al. (2011) ignored these candidates. Klein and Haubold (2007) discussed Rotodactylus tracks, attributing them to a Lagosuchus (Marasuchus)-grade of stem dinosaurs, despite differences in relative toe length and ignoring earlier published matches to Cosesaurus (Peters 2000), as blogged earlier.
Brusatte et al. (2011) also presented Sphingopus, a much larger ichnite created by a rauisuchid. The distal phalanges were shorter and pedal digit 4 was shorter than 3. Note digits 2-4 bore most of the weight as in basal and theropod dinosaurs. Digit 5 was offset from the others by a large hook. See more on functionally tridactyl pedes here.
Since two derived paraornithischian dinosaurs, Lotosaurus (and perhaps Ctenosauriscus) are from the latest early Triassic, somewhere on planet Earth dinosaur stem taxa were taking their first bipedal steps earlier in the Early Triassic. Asilisaurus (Nesbitt et al. 2010) was a contemporary. With so much variation occuring in the relatively short Olenekian (early Triassic) period, the middle and late Triassic now appear to be, by comparison, a time of relative evolutionary stasis and geographical radiation in dinosaur evolution.
Then There’s the Late Arrival of Dinosaur Precursors…
Pseudhesperosuchus and Trialestes phylogenetically nest before dinosaurs, but chronologically nest after them. We can only assume that both represented late surviving lineages of earlier radiations, part of the evolutionary stasis mentioned above.
Pedal digit 4 vs. 3
In basal reptiles pedal digit 4 was typically longer than 3. Generally, derived taxa with less than a sprawling configuration reduce digit 4 relative to 3. Exceptions include on the lepidosauromorph branch: 1. turtles; 2. Icarosaurus; 6. certain Jurassic and Cretaceous pterosaurs; and on the archosauromorph side: 7. certain higher placodonts; 8. certain higher pararchosauriforms with a reversal in Lagerpeton; , and 9. most Erythrosuchia (Erythrosuchus, Euparkeria and higher taxa). Digit 4 and 3 were subequal in drepanosaurs, langobardisaurs, therapsids, plesiosaurs and the Ornithosuchidae, among others.
Brusatte S, Niedźwiedzki G and Butler RJ 2011. Footprints pull origin and diversification of dinosaur stem-lineage deep into Early Triassic. Proceedings of the Royal Society of London, Series B, 278, 1107-1113.
Klein H and Haubold H 2007. Archosaur footprints – Potential for Biochronology for Triassic Contiental Sequences. In Lucas SG and Spielmann JA eds. The Global Triassic. New Mexico Museum of Natural History Science Bulletin 41. 120-130 online pdf
Nesbitt SJ, Sidor CA, Irmis RB, Angielczyk KD, Smith RMH and Tsuji LMA 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464 (7285):95-98. doi:10.1038/nature08718. PMID 20203608.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos 7:11-41.