Proceratosaurus: another theropod leaves the tyrannosauroids for the spinosauroids

This blogpost
continues a series of prior blogposts featuring the tested removal of taxa traditionally considered to be in the ancestry of Tyrannosaurus. Find those posts here, here, here and here. There were also a few blogposts that added non-traditional taxa to the lineage of tyrannosaurs. Find those here, here and here.

Figure 1. Only the lower 7/8 of the skull of Proceratosaurus is known. Here the skull in siitu, A tracing modified from Tracy Ford. And another tracing by Rauhut et al. 2010 with anterior and occipital views.

Figure 1. Only the lower 7/8 of the skull of Proceratosaurus is known. Here the skull in siitu, A tracing modified from Tracy Ford. And another tracing by Rauhut et al. 2010 with anterior and occipital views. Note the entire skull is concave ventrally, but the maxilla and jugal are both straight.

Proceratosaurus bradleyi
(Middle Jurassic, Bathonian, England, NHM R 4860, Fig. 1) was originally considered another Megalosaurus (Woodward 1910), then identified as a unique taxon, a likely ancestor of a another much larger and more robust horned theropod, Ceratosaurus (von Huene 1926). Those reports were made back in the day when there were very few theropods to compare with one another.

In more recent times,
Proceratosaurus was phylogenetically nested (according to Wikipedia, Angela Milner and the BBC, Rauhut et al. (2010), Loewen et al. (2013) and Brusatte et al. (2015)) as the earliest member of the tyrannosaur lineage (Figs. 2-4).

Figure 3. Theropods from Rauhut et al. 2010. Here Proceratosaurus, Dilong and Guanlong nest with tyrannosaurs.

Figure 2. Theropods from Rauhut et al. 2010. Here Proceratosaurus, Dilong and Guanlong nest with tyrannosaurs, but key taxa are missing.

The question is why do their trees and the large reptile tree differ?
The answer could be (once again) due to taxon exclusion and tradition. The shift in nestings could be due to the lack of more attractive sister taxa in traditional tyrannosaur studies. The large reptile tree includes those more attractive sister taxa. But that is not the complete answer in every case.

Figure 4. Tyrannosaurs from Brusatte et al. 2013. Some taxa nest elsewhere in the large reptile tree. Others are missing from this tree.

Figure 3. Tyrannosaurs from Brusatte et al. 2013. Some taxa nest elsewhere in the large reptile tree. Others are missing from this tree. Click to enlarge.

Could it be scoring?
I have not checked the scores and matrices in other studies. I do know that the sisters in the large reptile tree do share long lists of character traits, but D-shaped premaxillary teeth (often touted as a key trait restricted to tyrannosaurs) are not among the traits listed there. Did Spinosaurus and Suchomimus also have D-shaped premaxillary teeth? I don’t know. If not, could that trait be in their relatively short-snouted ancestors by convergence? At this point the answer is, apparently so.

Rauhut (2010 reported, “As close relationships of Proceratosaurus with several of the clades included in this analysis (coelophysoids, spinosauroids, and maniraptorans) have never been proposed previously, these clades were collapsed [individually] into [a] single operational taxonomic unit[s] (OTU[s]).” 

That’s a problem
as the large reptile tree found Proceratosaurus to nest closest to basalmost spinosauroids (former tyrannosauroids). Now do you see why it is SO important NOT to employ suprageneric taxa — ever! It is possible that Rauhut et al. (and those that followed) created their own problems by creating suprageneric taxa where they should not have done so. In Science you have to be open to any and all answers, without bias or a priori assumptions wherever practicable and possible. That’s why it is so convenient to start with the large gamut of possibilities provided by the large reptile tree (now 647 taxa and growing).

Figure 5. Theropods from Loewen et al. with pertinent taxa highlighted.

Figure 4. Theropods from Loewen et al. with pertinent taxa highlighted.

Proceratosaurus was added
to the large reptile tree (subset in Fig. 5) and it did not nest with tyrannosaurs, but with smaller Early Cretaceous taxa that traditionally nest with tyrannosaurs, but now nest with spinosaurs. Everyone agrees that Proceratosaurus nests with Guanlong and Dilong. Everyone else agrees that these three nested with tyrannosaurs (Figs. 2-4). So, I am the only unorthodox heretic at present.

Figure 2. The Dinosauria subset of the large reptile tree as of February 5, 2016. Here Proceratosaurus nests with several former long-snouted tyrannosaurs now closer to spinosaurs and allosaurs.

Figure 5. The Dinosauria subset of the large reptile tree as of February 5, 2016. Here Proceratosaurus nests with several former long-snouted tyrannosaurs now closer to spinosaurs and allosaurs.

The large reptile tree
provides ancestral taxa that share more traits (see below) with Late Cretaceous tyrannosaurs than the traditional putative Jurassic and Early Cretaceous candidates provided by the authors listed in the references. I promote these recovered candidates so they will be tested by others, as I have tested their candidate taxa.

Without a doubt,
the Late Cretaceous tyrannosaurs are all monophyletic. The question is, which taxa phylogenetically preceded them in the Early Cretaceous and Jurassic? Note that none of the taxon lists in any of the studies totally match one another. On the other hand, all of the studies are in general agreement. However, the recovered topologies don’t exactly match one another. And so the game is afoot.

Getting back to Proceratosaurus
Take a look at its sister on the allosaur branch in the large reptile tree: it’s Ceratosaurus. So maybe von Huene (1926) was on to something… or he was lucky.

Basic traits that Proceratosaurus, Guanlong and Dilong
share with Sinocalliopteryx, Deinocheirus and the spinosaurs.

  1. Long, low rostrum
  2. Sometimes smaller premaxillary teeth vs. maxillary teeth
  3. Tall orbit
  4. Premaxillary postero-lateral processes that may be present due only to the down tip of the naris.
  5. Ventral border of elongate naris formed by premaxilla + nasal
  6. Long, strongly recurved maxillary teeth
  7. Majority coverage of the quadrate by the squamosal and quadratojugal.
  8. Often, but not always, a nasal median crest.
  9. Often, but not always, a descending posterior skull relative to the maxilla

Given that
Sinocalliopteryx and Dilong had primitive feathers, all (except perhaps the giants) probably shared that rarely preserved trait. Given that the above nine traits are all skull traits, it is likely that this clade was trending toward a specific feeding niche, in this case, an aquatic one.

Someday this will all come together. 

References
Brusatte SL and Carr TD 2016. The phylogeny and evolutionary history of tyrannosauroid dinosaurs. Nature, Scientifice Reports 6 (8 pages), 20252; doi: 10.1038/srep20252.
Loewen MA, Irmis RB, Sertich JJW, Currie PJ, Sampson SD 2013. Tyrant Dinosaur Evolution Tracks the Rise and Fall of Late Cretaceous Oceans. PLoS ONE 8(11): e79420. doi:10.1371/journal.pone.0079420
Rauhut OWM, Milner AC and Moore-Fay S 2010. Cranial osteology and phylogenetic position of the theropod dinosaur Proceratosaurus bradleyi(Woodward, 1910) from the Middle Jurassic of England. Zoological Journal of the Linnean Society, published online before print November 2009. doi:10.1111/j.1096-3642.2009.00591
von Huene F 1932. Die fossile Reptil-Ordnung Saurischia, ihre Entwicklung und Geschichte. Monographien zur Geologie und Palaeontologie (Serie 1), 4: 1–361.
Woodward AS 1910. On a Skull of Megalosaurus from the Great Oolite of Minchinhampton (Gloucestershire). Quarterly Journal of the Geological Society 66: 111–115.

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