Meet the mother (or father or sister) of all dinosaurs!

PVL 4597 is the closest thing we now have to a dinosaur ancestor
Earlier we looked at PVL 4597 (Lecuana and Desojo 2012), a taxon attributed to Gracilisuchus (Romer 1972). Inspired to take another look at it, I did so and recoded several characters (Fig. 1). The large reptile tree (Fig. 2) now nests PVL 4597 at the base of Trialestes + the Dinosauria. Trialestes may be slightly closer to dinosaurs, but it is known from less published material and much of it broken up.

No doubt PVL 4597 is a sister to Gracilisuchus near the base of the Archosauria
It’s also a bigger sister with a perforated acetabulum, the beginnings of a truncated anterior ilium, a ventrally and slightly posteriorly directed pubis, with metatarsals 3 and 4 of nearly equal length and pedal digit 5 with one phalanx without a ginglymal joint (for the reception of the next phalanx). These are traits PVL 4597 shares with certain basal dinosaurs in the large reptile tree. Prior to the nesting of PVL 4597, Gracilisuchus was our closest known dinosaur ancestor, but it also nested more firmly with basal crocs, as it continues to do (Fig. 2).

PVL 4597 is not a dinosaur,
but among the 330+ taxa in the tree, it is extremely close to them. This is how dinosaurs (and by extension birds) began.

Addendum figure. This is a better rendition of the new hindquarters of the PVL specimen. Apologies for any earlier inaccuracies. Thanks to M. Mortimer for pointing out I had mistaken a right ischium for a left one and tarsal inaccuracies. If there are further problems, please let me know, or contact Romer.

Figure 1. Until further notice, here is the most basal dinosaur now known and a descendant to a sister of Gracilisuchus. Purple is the partially perforated (no embayment of the ilium) acetabulum. Green areas are reconstructed from broken edges.

Perforated or Not?
According to Lecuana and Desojo (2012), “On the basis of the preserved region of the acetabulum, it can be inferred that it was not perforated…” Unfortunately, given the pieces to work with, they just don’t fit more closely together unless the scale bars are wrong. According to Lecuana and Desojo (2012), “The absence of a discrete ischiatic surface contrasts with the ancestral condition of Archosauromorpha” and “an articular surface for the pubis cannot be seen anteriorly.” Here (fig. 1) the hypothetical connection is short, small and weak, with very little articular surface between the pubis and ischium. And, of course, this reconstruction produces the semi-perforate acetabulum. The lack of an articular surface on the ischium refers to a pubic articulation. Otherwise it is clear that the proximal portion of the ischium is semi-circular for the anchoring of the rotating femur.

 Subset of the large reptile tree focusing on the Archosauria = crocs + dinos.

Figure 2. Subset of the large reptile tree focusing on the Archosauria = crocs + dinos. Click to see the rest of the tree.

Ankle Joints 
Like Gracilisuchus, PVL 4597 retains a “crocodile-normal” ankle joint. This is close to, but not quite, the mesotarsal simple-hinge joint we see in Herrerasaurus, in which the astragalus has an ascending process and is twice the width of the calcaneum, which does not have a tuber. However, the importance of this trait is mitigated by the nesting of poposaurs deep within the Dinosauria, at the base of the Phytodinosauria. Poposaurs have a similar sort of “croc-normal”  ankle subequal astragalus and a calcaneum with a tuber, but this sort of ankle may have developed independently and by convergence, as in crocodylomorphs, as we discussed earlier.

This ankle is not a conventional dinosaurian ankle. No, the mesotarsal ankle with the small calcaneum without a tuber evolved later. Then, apparently, the old-style ankle returned in certain poposaurs.

Like Gracilisuchus the femoral head is inturned but not rectangular or sharply inturned. The fourth trochanter was weakly developed. Both of these taxa were just getting into bipedal locomotion. A large, sharp fourth trochanter is common in dinosaurs.

Like Gracilisuchus, PVL 4597 had relatively short legs with a tibia shorter than the femur and the tibia less than twice the ilium length. The latter is a trait shared with dinosaurs other than theropods.

Like Gracilisuchus the proximal metatarsals of PVL 4597 were of subequal width, although mt1 was the narrowest. In basal dinosaurs mt1 and mt5 are narrower than the middle three metatarsals. Prosauropods and sauropods reverse this pattern and re-widen metatarsals 1 and 5.

Like Gracilisuchus there is a series of paramedian osteoderms in PVL 4597. 

More to come
There is more to PVL 4597 yet to be published, according to Lecuana and Desojo (2012). Earlier we noted the similarities in the skull of Gracilisuchus and Herrerasaurus. PVL 4597 further cements this relationship recovered by the large reptile tree.

Stance
Earlier workers (Romer 1972) wondered if Gracilisuchus was a biped or not. Close taxa, including dinosaurs like Herrerasaurus and Scleromochlus were both bipedal. Ancestors were not bipedal (although the derived rauisuchians, Smok and Postosuchus, gave it a shot by convergence).

Expanded costal plates
The expanded costal plates of Gracilisuchus are autapomorphic (no ancestors and few descendants (but see Hesperosuchus) have them). They strengthen the rib cage and play a role in respiration. Their appearances elsewhere in the tetrapod tree include Ichthyostega, an early tetrapod, Thrinaxodon, a cynodont, along with many birds, (like Ichthyornis) and velociraptors with their uncinate processes. Crocs have unossified uncinate processes.

Conventional Thinking
Most paleontologists (e.g. Nesbitt 2011, Irmis et al. 2007) hold that the odd biped Lagerpeton and pterosaurs were part of the heritage of dinosaurs. The large reptile tree does not support that hypothesis. Lagerpeton nests with the pararchosauriformes and pterosaurs nest with tritosaur fenestrasaur lizards.

Defining the Dinosauria
Dinosaurs are defined as the clade consisting of TriceratopsNeornithes [modern birds], their most recent common ancestor (MRCA), and all of its descendants (Benton 2004). Since PVL 4597 and Trialestes nest below the split between the ancestors of birds and Triceratops, so they are the last “common” ancestors now known. They are not dinosaurs, but nest just outside the Dinosauria proper.

Synapomorphies of Dinosaurs
From Wiki/Dinosaur, based on Nesbitt 2011, dinosaurs share the following traits. These are all either unknown or not found in PVL 4597 or Gracilisuchus. Boldface traits are preserved in the published version of PVL 4597. Once again only one and maybe two traits can be found in PVL 4597 and Gracilisuchus, so they nest outside the Dinosauria proper.

  • exocciptials do not meet along the midline
  • fossa (dip in the bone) frames the upper temporal fenestra – Gracilisuchus has this, but minimally.
  • epipophyses present in anterior cervical vertebrae 3-5
  • apex of deltopectoral crest 1/3 down the humerus
  • radius length less than 80% of humerus
  • fourth trochanter on the femur is a sharp asymmetrical flange – not on PVL 4597
  • the articular facet for the fibula on the astragalus and calcaneum is less than a third the width – not on PVL 4597
  • proximal articular surfaces of the ischium with the ilium and the pubis separated by a large concave surface – present on PVL 4597
  • cnemial crest on the tibia arcs anterolaterally – not on PVL 4597
  • proximodistally-oriented ridge on the posterior face of the distal tibia  – not on PVL 4597

It should be remembered that Nesbitt (2011) did not consider poposaurs and their kin to be dinosaurs, so that affects this list. We discussed this problem earlier.

And check out that pubis!
The pubis of PVL 4597 was vertically oriented, maybe a little skewed toward the back. This is exactly what we are looking for in an ancestor to Herrerasaurus and Panphagia and Ornithischia. And yes, I think the pubis evolved both ways from this primitive configuration: sometimes forward, sometimes backward. PVL 4597 represents the starting point.

In Summary
This situation is like that of the rhipidistian fish on the family tree of vertebrates. Sure they acted like fish and looked like fish, but they had a few traits that tell us they are the ones from which tetrapods arose. Same with PVL 4597. Archosaurs of this type ultimately begat dinosaurs. And yes, that means we’re still looking for that basal dinosaur that fulfills all of the traits on the Nesbitt (2011) checklist and still nests close to PVL 4597.

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.

References

Benton MJ 2004. Origin and relationships of Dinosauria. In Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 7–19.
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
Lecuona A and Desojo  J B 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 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.
wiki/Gracilisuchus 

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