Earlier we looked at the origin of dinosaurs. Here is an update.
Figure 1. The origin of dinosaurs as portrayed by the phylogenetic series of closest known sister taxa to the actual unknown undiscovered true lineage of dinosaurs. A. Youngina, B. Proterosuchus, C. Garjainia, D. Euparkeria, Vjushkovia, 1. Ticinosuchus, 2. Yarasuchus, 3. Decuriasuchus, 4. Turfanosuchus, 5. Gracilisuchus, 6. PVL 4597, 7. Trialestes, 8. Herrerasaurus, 9. Marasuchus, 10. Daemonosaurus.
Notably absent here are Lagerpeton, phytosaurs and pterosaurs — because they don’t belong. They were added by other studies (Brusatte et al. 2010, Nesbitt 2011) because they were following traditions and did not have a large gamut reptile study, like we do, to tell them those taxa are not closely related or in the lineage of dinosaurs.
The story here goes back to the basal archosauriform Youngina, but it could have gone back to the basal tetrapod, Ichthyostega.
A. Archosauriformes – Youngina
Derived from basal diapsids like Aphelosaurus, Thadeosaurus and basal protorosaurs (in that order), Youngina nests at the base of the Archosauriformes. This clade includes the Pararchosauriformes and Euarchosauriformes. A taller skull and a nascent antorbital fenestra without a fossa on a generalized neck and body mark Youngina.
B. Euarchosauriformes – Proterosuchus
Probably more aquatic, like a crocodile, Proterosuchus is derived from Youngina with a longer, relatively smaller skull, a longer neck and a droopy snout. The cervicals were taller. The scapula was more robust. The ilium was lower with a small anterior process. The pubis and ischium were in complete contact and oriented medially. Metatarsal 5 was reduced and hook-shaped. Pedal 1.1 was aligned with metatarsals 2 and 3.
C. Erythrosuchia – Garjainia
Possibly aquatic, like a hippopotamus, Garjainia is also derived from Youngina, largely skipping the proterosuchid stage.
D. Euparkeriamorpha – Euparkeria
So, the big question is, is Euparkeria a tiny erythrosuchid? Or just another tiny derived younginid that begat giant erythrosuchids, ornithosuchids and rauisuchids? Not sure yet. We need a few more euparkeriids. Probably terrestrial.
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.
E. Rauisuchia – Vjushkovia
Probably terrestrial, overall Vjushkovia had the proportions of Euparkeria, only larger. It didn’t have the giant skull and short tail of erythrosuchids. With larger size comes larger prey. From Vjushkovia we get a mixed bag of rauisuchians that includes a variety of sometimes long, necked, sometimes finbacked and long-necked, sometimes herbivorous and the slender-limbed protoarchosaurs. So this is a key taxon.
1 and 2. Ticinosuchia – Ticinosuchus and Yarasuchus
This mixed bag of taxa includes some sail-back carnivores, long-necked fish-eaters and armored plant-eaters. Ticinosuchus is basal to the armored plant-eaters, so is quite far off the basal line, better represented by Decuriasuchus (noted below), which looks more like a rauisuchid.
3. Protoarchosauria – Decuriasuchus
This mixed bag of taxa includes some long torso, short legged carnivores, like Decuriausuchus, some short-torso long-legged bipedal carnivores like Pseudhesperosuchus and a tiny likely biped, Lewisuchus. Decuriasuchus had an elevated neck with low neural spines, gracile forelimbs and short hindlimbs. Such a gracile pectoral girdle is also seen in the probably biped Arizonasaurus at the base of the Ticinosuchia (above).
4. Archosauria – Turfanosuchus – Poposauridae
This odd taxon appears here at the base of the Archosauria. Turfanosuchus looks like a smaller decuriasuchid with a smaller skull and longer neck. The torso is shorter. The tail is more gracile. Other poposaurs also nest with Turfanosuchus.
5. Crocodylomorpha – Gracilisuchus
The torso and tail continue to shrink in Gracilisuchus, nesting at the base of the Crocodylomorpha, a clade with several basal bipedal members. Here the skull is wider than tall.
6 and 7. Dinosauria – Trialestes and PVL 4597
Known from bits and scraps, Trialestes and PVL 4597 are all that we now know of the base of the Dinosauria. So there’s some mystery here! Who knows if they are bipedal or not?? Both taxa are rather mid-sized to small. PVL 4597 includes a calcaneal tuber and a dinosaurian semi-perforate pelvis. Metatarsals 3 and 4 are subequal. Trialestes retains elongated proximal carpals and had long curved teeth.
8 and 9. Theropoda – Herrerasaurus and Marasuchus
Two short-torso, long hind limb bipeds are basal dinosaur theropods. The tail is deep proximally, attenuated distally. The short torso enables the feet to be placed beneath the center of gravity, freeing the forelimbs. The neck is more gracile and probably more flexible. The dorsal vertebrae are deepest near the sacrum, not so deep both fore and aft.
10. Phytodinosauria and Ornithischia – Daemonosaurus
Several basal dinosaurs, like Pampadromaeus and Panphagia, developed a smaller head and longer neck. Daemonosaurus continued that trend toward the Ornithischia. Neural spines are greatly reduced.
Remarkable as it may seem the current chronology of the ancestry of the Dinosauria took place all within the comparatively brief Early Triassic, based on the Late Permian appearance of Youngina and the early Middle Triassic appearance of Lotosaurus, a derived poposaurid dinosaur. Fossils of these dino ancestors, most of which come from later epochs, evidently reflect derived and late-surviving taxa, when root stocks were much more widespread and thus easier to find as fossils.
The Early Triassic appearance of Euparkeria, when it was likely widespread, may represent a Late Permian origin and diversification.
We have a Middle Triassic appearance of Ticinosuchus, itself a derived rauisuchian, but that means its more plesiomorphic ancestors, and the more direct ancestors of dinosaurs, likely appeared and diversified in the Early Triassic.
The basal dinosaurs, Herrerasaurus and Marasuchus are likewise Middle Triassic, with origins likely much earlier.
Brusatte SL , Benton MJ , Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.