The Protodinosauria and the Origin of the Dinosauria

The large reptile tree recovers the following taxa at the base of the Archosauria leading toward the Dinosauria (Fig. 1). This is an update of prior posts on dinosaur origins with PVL 4597 moving from closer to Trialestes to between Gracilisuchus and Lewisuchus.

  1. Gracilisuchus
  2. PVL 4597 (the Tucuman specimen attributed to Gracilisuchus)
  3. Lewisuchus
  4. Pseudhesperosuchus, Carnufex and Junggarsuchus
  5. Trialestes
  6. and finally, the basal dinosaur, Herrerasaurus.

Pterosaurs and Lagerpeton nest elsewhere. They are not part of the dino lineage. Marasuchus, which often nests outside the Dinosauria in other trees, nests with a few other odd theropods here.

Figure 1. Origin of the dinosaurs and protodinosaurs. Here Gracilisuchus, at the base of the Crocodylomorpha and Archosauria, is basal to the PVL 4597 specimen attributed to Gracilisuchus, Lewisuchus and Pseudhesperosuchus, taxa leading to Herrerasaurus at the base of the Dinosauria.

Figure 1. Origin of the dinosaurs and protodinosaurs. Here Gracilisuchus, at the base of the Crocodylomorpha and Archosauria, is basal to the PVL 4597 specimen attributed to Gracilisuchus, Lewisuchus and Pseudhesperosuchus, taxa leading to Herrerasaurus at the base of the Dinosauria. Click to enlarge.

Here (Fig.1) crocs and dinos have a last common ancestor
close to Gracilisuchus, probably in the Middle Triassic. Both started small and bipedal. Crocs had a wider skull. Dinos had a narrower skull. The reduction of the calcaneal tuber occurred in parallel. The tuber redeveloped in extant crocs.

Prior to these taxa
are larger forms, including Decuriasuchus and the basal poposaur, Turfanosuchus. So once again, phylogenetic miniaturization is key to the origin of both crocs and dinos (together, the Archosauria).

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.

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

Figure 3. Family tree of the Archosauria and basal Dinosauria. Bootstrap scores are shown.

 

 

 

 

Wikipedia reports, Paleontologists think that Eoraptor (Fig. 2) resembles the common ancestor of all dinosaurs;[ if this is true, its traits suggest that the first dinosaurs were small, bipedal predators. The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators. Dinosaurs may have appeared as early as 243 million years ago, as evidenced by remains of the genus Nyasasaurus from that period, though known fossils of these animals are too fragmentary to tell if they are dinosaurs or very close dinosaurian relatives.”

Too bad they are so tentative at Wikipedia when the large reptile tree lays it out pretty clearly. The purported and popular clade, “Ornithodira,” is, of course, not supported by the large reptile tree.

References
Benton MJ and Clark JM 1988. Archosaur phylogeny and the relationships of the Crocodilia in MJ Benton (ed.), The Phylogeny and Classification of the Tetrapods 1: 295-338. Oxford, The Systematics Association.
Bittencourt JS, Arcucci AB, Maricano CA and Langer MC 2014. Osteology of the Middle Triassic archosaur Lewisuchus admixtus Romer (Chañares Formation, Argentina) its inclusivity, and relationships amongst early dinosauromorphs. Journal of Systematic Palaeontology. Published online: 31 Mar 201. DOI:10.1080/14772019.2013.878758
Bonaparte JF 1982. Classification of the Thecodontia. Geobios Mem. Spec. 6, 99-112
Bonaparte JF 1969. Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.
Clark JM, Sues H-D and Berman DS 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20(4):683-704.
Clark JM et al. 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20 (4): 683–704. doi:10.1671/0272-4634(2000)020[0683:ANSOHA]2.0.CO;2.
Clark JM, Xu X, Forster CA and Wang Y 2004. A Middle Jurassic ‘sphenosuchian’ from China and the origin of the crocodilian skull. Nature 430:1021-1024.
Juul L 1994. The phylogeny of basal archosaurs. Palaeontographica africana 1994: 1-38.
Lecuona A and Desojo, JB 2011. Hind limb osteology of Gracilisuchus stipanicicorum(Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2): 105–128.
Nesbitt SJ. et al. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464(7285):95-8
Parrish JM 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology 13(3):287-308.
Reig, OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna; XIV, Lewisuchusadmixtus, gen. et sp. nov., a further thecodont from the Chañares beds. Breviora 390:1-13

wiki/Gracilisuchus
wiki/Lewisuchus
wiki/Pseudhesperosuchus
wiki/Trialestes

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