Cau 2018: Evolution of the avian body plan – part 4

We looked at Cau 2018
from a different 3-part perspective earlier here, here and here.

Dr. Andrea Cau 2018 recovers a cladogram of the Theropoda
(Figs. 1-4) focusing on the ultimate appearance of extant birds, represented by Meleagris, the extant turkey (Fig. 4). Many errors accrue in Cau 2018 due to simple taxon exclusion, as you’ll see below. Many clades are not valid when tested in the LRT.

Figure 1. The base of the Cau 2018 cladogram. White boxes are clades that agree with the LRT. Light blue taxa are members of the Phytodinosauria. Yellow taxa are theropods. Red lines indicate invalidated clades. Continues on figure 2.

Figure 1. The base of the Cau 2018 cladogram. White boxes are clades that agree with the LRT. Light blue taxa are members of the Phytodinosauria. Yellow taxa are theropods. Red lines indicate invalidated clades. In the LRT bipedal Crocodylomorpha and Poposauria are successively more distant outgroups to the Dinosauria. Sacisaurus, Silesaurus and Asilisaurus are poposaurs in the LRT. Dromomeron and Lagerpeton are chanaresuchids.  Continues on figure 2.

First of all,
Cau 2018 uses the wrong outgroup taxa (taxon exclusion). In the large reptile tree (LRT, 1327 taxa), which tests all of Cau’s outgroup taxa, the outgroup for the Dinosauria is Crocodylomorpha, specifically the basal bipedal crocs,  represented by one taxon in Cau 2018, Lewisuchus. Only in the LRT does Archosauria include just Crocodylomorpha and Dinosauria.

Invalidated clades:

  1. Pan-Aves – all animals more closely related to birds than to crocodiles (traditionally = Avemetatarsalia) includes pterosaurs, so this clade is a junior synonym for Reptilia. In the LRT, Pan-Aves is a junior synonym for Dinosauria.
  2. Dinosauromorpha – the last common ancestor (LCA) of Lagerpeton, Marasuchus, Pseudolagosuchus and the Dinosauria. In the LRT this is a junior synonym for Archosauriformes –
  3. Dinosauriformes – Originally: Herrerasaurus + Dinosauria. Currently Marasuchus, Silesauridae and the Dinosauria. Silesaurus is a poposaur, nesting outside the Archosauria. So this is an awkward name that includes Poposauria + Crocodylomorpha + Dinosauria in the LRT.
  4. Dracohors – Practically: Silesauridae and the Dinosauria. Definition: “Most inclusivemost inclusive clade containing Megalosaurus, but excluding Marasuchus“(Cau 2018). Same issues arise as in Dinosauriformes. Cau did not include basal bipedal crocs (Fig. 1), which clarify basal dinosaur origins and relations in the LRT.

Also unfortunately Cau 2018 excluded several basal theropods.
Segisaurus and Procompsognathus were not included. These taxa attract the traditional stem-dinosaurs Marasuchus and Guaiabasaurus to this clade. Zuolong nests as a relative to the phytodinosaur, Chilesaurus in the Cau cladogram because the LRT sisters to the basal ornithischian, Chilesaurus, were not included. Adding basal phytodinosaurs shifts Chilesaurus out of the Theropoda and shifts Zuolong to the base of the Marasuchus clade. Pisanosaurus and Daemonosaurus also nest within the Ornithischia in the LRT with taxon inclusion.

Figure 2. From Cau 2018, focusing on basal Theropoda. Taxa also present in the LRT are boxed in white.

Figure 2. From Cau 2018, focusing on basal Theropoda. Taxa also present in the LRT are boxed in white. In the LRT Limusaurus nests with oviraptorids. Masiakasaurus nests in the Tyrannosaurus clade. Coelophysis and Dilophosaurus nest together.  Continues on figure 3.

At the next stage in the Cau 2018 cladogram
(Fig. 2) and in the LRT, Dilophosaurus and Coelophysis nest as basal theropods. Where they differ, Sinocalliopteryx nests at the base of this clade in the LRT, but as the outgroup to the Tyrannoraptora in Cau 2018 (Fig. 3).

Invalidated clades:

  1. Averostra – CeratosaurusAllosaurus , their last common ancestor and all its descendants (Ezcurra and Cuny 2007) is not in the lineage of birds in the LRT. 
  2. Tetanurae – All theropods more closely related to modern birds than to Ceratosaurus. This is a junior synonym of Compsognathidae Cope, 1871 in the LRT. The traditional compsognathids, Sinocalliopteryx, Scipionyx and two Compsognathus taxa nest at basal leaves of several basal theropod clades in the LRT.
Figure 3. Tetanurae. Continues on figure 4.

Figure 3. Tetanurae. Continues on figure 4.

At the next stage
(Fig. 3) Cau nests Sinocalliopteryx with the large Compsognathus. By contrast, the LRT nests Sinocalliopteryx basal to Dracoraptor, Zupaysaurus and Megapnosaurus (taxa excluded by Cau) on one branch, Scipionyx (also omitted by Cau) + Ceratorsauria on the other. To shift Sinocalliopteryx to the large Compsognathus adds 24 steps to the MPT.

Validated clades:

  1. Coelurosauria – theropods more closely related to birds than to carnosaurs, includes Aorun, compsognathidstyrannosaursornithomimosaurs, and maniraptorans. The LRT also recovers this clade.

Invalidated clades:

  1. Tetanurae – (defined as: theropods more closely related to modern birds than to Ceratosaurus). Cau 2018 wrongly includes the basal ornithischian, Chilesaurus at the base of this clade. Cau 2018 also includes Allosaurus and Eustrespondylus in this clade. These nest with Ceratosaurus in the LRT. In any case, as defined, this is a junior synonym of Coelurosauria.
  2. Neotetanurae – defined as: containing Allosauroidea and Coelurosauria, and excluding other tetanurans such as megalosauroids, Same problem as Tetanurae.
  3. Tyrannoraptora – includes tyrannosaursornithomimosaurs, and maniraptorans. In the LRT compsognathids nest at the bases of all these taxa, so this nominal clade is polyphyletic without compsognathids.
Figure 3. Theropods in the lineage of birds modified from Cau 2018.

Figure 4. Theropods in the lineage of birds modified from Cau 2018. Continues on figure 5.

At the next stage
(Fig. 4) Cau 2018 xxx

Validated clades:

  1. Averaptora – Paraves sans Troodontidae + Dromaeosauridae (Cau 2018). The LRT validates this clade (Xiaotingia + Anchiornis + birds)

Invalidated clades:

  1. ManiraptoriformesOrnithomimus + birds, LCA and all descendants (Holtz 1995). In the LRT this clade is a junior synonym for Compsognathidae.
  2. Maniraptora – Sans Ornithomimosauria, this clade is also a junior synonym.
  3. Pennaraptora – The LCA of OviraptorDeinonychus and Passer and all descendants. In the LRT this clade is a junior synonym for Compsognathidae.
  4. Paraves – All theropods more closely related to birds than to oviraptorosaurs. Traditionally this includes Scansoriopterygidae at a basal node. In the LRT the LCA  is Mirischia and more complete Ornitholestes. Scansoriopterygidae nests within Aves in the LRT when all the Solnhofen birds are included in the taxon list. Only one, Archaeopteryx, is included in Cau 2018.
  5. Avialae – Theropods closer to birds than to deinonychosaurs. In Cau 2018 this deletes three scansoriopterygid birds and Xiaotingia. In the LRT Xiaotingia, Ostromia and Eosinopteryx are the proximal outgroup clade to birds.
  6. Pygostylia – intended to encompass all avialans with a short, stubby tail (Chatterjee 1997), but this is a trait that appears several times by convergence. Late defined as the LCA of Confuciusornis + Neornithes. Both are descendants of Archaeopteryx (Wellnhoferia) grandis, which has a long tail and lacks a pygostyle. This confusion in Cau 2018 (and all other bird workers) comes from the traditional exclusion of other Solnhofen birds from analysis. This is where all Cretaceous bird clades had their radiation documented.
Figure 5. Bird subset from Cau 2018.

Figure 5. Bird subset from Cau 2018.

In the final subset of the Cau 2018 cladogram
(Fig. 5) several of the toothed euornithes are wrongly separated from the other toothed Cretaceous birds in figure 4. Patagopteryx nests with missing Struthio in the LRT. Once again, taxon exclusion is the reason for problems in the Cau study. 

Validated clades:

  1. Aves – extant birds (includes Cretaceous toothed birds in the LRT)

Invalidated clades:

  1. Ornithothoraces – (= Enantiornithes + Euornithes) in the LRT these two clades comprise the first split within Aves and both include a specimen of Archaeopteryx at the base. So this is a junior synonym of Aves.
  2. Ornithuromorpha – (= Euornithes, or the most recent common ancestor of all avialans closer to modern birds than to Sinornis.) Since Sinornis is an enantiornithine, this is also a junior synonym of Aves.
  3. Carinatae – (= all birds and their extinct relatives to possess a keel, or “carina”, on the underside of the breastbone used to anchor large flight muscles.) Alternatively, as in Cau 2018, Ichthyornis + extant birds. In the LRT Ichthyornis nests within extant birds.
  4. Ornithurae – (the LCA of Ichthyornis, Hesperornis and extant birds) In the LRT Ichthyornis and Hesperornis and related Cretaceous toothed birds nest within the clade of extant birds.
Figure 1. More taxa, updated tree, new clade names.

Figure 6. Subset of the LRT focusing on theropod dinosaurs.

Phylogenetic analysis
is indeed key to understanding the assembly of the avian body plan. Taxon inclusion is key to any phylogenetic analysis. Unfortunately for Cau 2018, so many taxa were excluded that Cau recovers a less than optimal origin for the Dinosauria, Theropoda and Aves, along with a series of false positive clades, as explained above.

References
Cau A 2018. The assembly of the avian body plan: a 160-million-year long process. Bollettino della Società Paleontologica Italiana, 57 (1), 2018, 1-25. Modena

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