Clavicles, the furcula, and what’s going on in basal archosaurs

Updated within 24 hours of this post with a look at Vickaryous and Hall 2006, which just came to my attention. They consider the possibility of homologizing the interclavicle and furcula. See below.

Today’s post
comes from a paper on clavicles and furculae by Bryant and Russell (1993). The question was: since most theropods, and most dinosaurs, do/did not have clavicles, is the furcula of birds a neomorph (new structure)? Or is this the reappearance, after a phylogenetic gap, of topologically identical clavicles? The question becomes more complex with the occasional appearance of clavicles in the Dinosauria.

Nesbitt et al. 2009
updated the furcula issue. They write, “Given this absence of clavicles and interclavicle in dinosaurian outgroups, the homology of the furcula to other components of the shoulder girdle has been contentious (Bryant and Russell, 1993). This debate is based largely on absence of evidence and will be explored more fully latter in this article.” They continued, Additionally, topographic connectivity of the furcula to the other pectoral girdle elements in avian and other theropod dinosaurs is entirely consistent with and supportive of homology of the avian clavicle with the ancestral reptilian and tetrapod clavicle.” 

Of course, the Nesbitt et al. outgroups
are not the same outgroups in the LRT.

Before we start
Bryant and Russell followed the invalidated tradition of including pterosaurs with dinosaurs in the outmoded clade, Ornithodira. Worse than that, they and a number of high-profile paleontologists before them believed that pterosaurs had neither clavicles nor an interclavicle. Wild 1993 demonstrated that the pterosaur sternal complex is comprised of fused clavicles, interclavicle and sternum. These are separate in the pterosaur ancestor, Cosesaurus. Nesbitt et al. 2009 acknowledged that observation by Wild 1993 and agreed on the fusion of the sternal and clavicle elements in pterosaurs, However they followed in the wake of the invalid Ornithodira hypothesis.

In the large reptile tree
(LRT, 1012) clavicles that are mediallly broad are present in fish and basal tetrapods. That’s where we start. The situation changes in:

  1. Frogs and kin – clavicles are medially narrow
  2. The Tuditanus clade – clavicles are medially narrow
  3. Pantylus – clavicles are medially narrow
  4. The Microbrachis clade – clavicles are medially narrow
  5. Within the Reptilia/Lepidosauromorpha: the Milleretta clade (remaining Lepidosauromorpha – clavicles are medially narrow,
  6. Except the Caseasauria and except Turtles, where clavicles become part of the plastron
  7. And except Mecistotrachelos clade where the clavicles are absent
  8. and except Longisquama + Pterosauria where the clavicles are fused to the sternal complex
  9. and except Tetrapodophis + snakes.
  10. Within the Reptilia/Archosauromorpha: the Diplovertebron/Romeriscus clade – clavicles are medially narrow
  11. The Anomodontia – clavicles are medially narrow
  12. Titanophoneus + Cynodontia (including mammals) – clavicles are medially narrow
  13. Except in higher Carnivora – clavicles are absent
  14. Except in Odontoceti – clavicles are absent
  15. Except in Phenacodontidae (includes all hoofed mammals)
  16. Prodiapsida – clavicles are medially narrow except Petrolacosaurus where clavicles are medially broad.
  17. Except Nothosaurus – clavicles are medially broad
  18. Except the Simosaurus clade (plesiosaurs) – clavicles are medially broad
  19. Within basal Younginiformes (including Archosauriformes) – clavicles are medially narrow
  20. Except in certain Rauisuchids, Decuriasuchus, where clavicles are provisionally absent.
  21. Clavicles are present in the poposaurs, Poposaurus and Lotosaurus – except TurfanosuchusSilesaurusShuvosaurus + Effigia (derived poposaurs) where clavicles are provisionally absent.
  22. Except Crocodylomorpha (crocs) – clavicles are absent
  23. In basal Dinosauria and Prodinosauria the pectoral region is not well preserved (see below)
  24. Except possibly in Junggarsuchus (Fig. 1) – tiny clavicles may be present
  25. Within Orinithischia: Psittacosaurus – clavicles are medially narrow (neomorph)
  26. Within Sauropodomorpha: Massospondylus – clavicles are medially narrow (neomorph?)
  27. Segisaurus, Coelophysis and higher theropods – furcula is present, as in birds, but lost in ornithomimosaurs and several other derived theropod clades 

The furcula and the absence of clavicles
Then furcula goes back to basal theropods, but is lost in certain theropod clades, like Ornithomimosauria. In basal theropods it appears to be  a neomorph without direct antecedent. At present, as Newbitt et al. noted, in basal Archosauria (all crocs and basal dinos) the clavicles have not been found. In general the clavicle appears to be lost in taxa that are preserved incomplete and scattered.

But all is not lost…
If we could only find a clavicle in Lewisuchus,. Gracilisuchus, Junggarsuchus, Herrrerasaurus and Tawa we would have a more or less continuous clavicle presence from fish to birds in the LRT. These taxa need to either have a set of clavicles, or some excuse for not preserving them. The latter appears to be the case often enough, as demonstrated here:

  1. Lewisuchus – incomplete and jumbled specimen in which the clavicles could have been washed away.
  2. Gracilisuchus – pertinent area lost during excavation
  3. Junggarsuchus – appears to be minimally and tentatively present (Fig. 1), but really,  who knows what that little green bone is?
  4. Herrerasaurus and Sanjuansaurus – pertinent area lost during excavation
  5. Tawa – represented by “two nearly complete skeletons and several other partial specimens collected in a tightly associated small grouping at a single locality.” but no clavicle was reported and no in situ images were published suggesting that the skeletons were disassociated. Moreover, any out-of-place clavicles could be mistaken for ribs.
  6. Eodromaeus – forelimbs and pectoral girdle missing from holotype.
  7. Eoraptor – clavicles were not found and the pectoral girdle has taphonomically shifted.

In summary,
the data is largely missing from the transitional taxa at and near the base of the Archosauria. So there’s still hope that the clavicles were present in these taxa and will someday be discovered among more complete fossils.

Figure 2. Junggarsuchus and its overlooked clavicle. Let's consider this provisional until confirmed.

Figure 1. Junggarsuchus and its overlooked clavicle. Let’s consider this provisional until confirmed.

Finding overlooked traits
For decades it was thought that no dinosaurs had a furcula or even clavicles. As it turns out, at least in Theropoda, they were largely overlooked. Nesbitt et al. 2009 write: “Furculae occur in nearly all major clades of theropods, as shown by new theropod specimens from the Early Cretaceous of China and a close inspection of previously collected specimens.” Finding overlooked traits is something we should all be doing. Some of these turn out to be important.

On a similar note…
earlier we looked at the homology of the central bones of the wrist and their migration to the medial rim in pterosaurs and pandas, and the previously overlooked evidence for that.

Update
Vickaryous and Hall 2006 employed embryology to determine that the Alligator interclavicle is equally parsimonious as a homolog of the Gallus furcula. They note:

  1. “the lateral processes (of the interclavicle in Alligator) are lost yielding a flattened bar-like element.” Actually that I-shaped bar goes back to basal crocodylomorphs.
  2. “At no time during skeletogenesis (in Alligator) are there any signs of any developmental stages of clavicles or clavicular rudiments, nor are there any signs of cartilage (primary or secondary).” — this statement speaks for itself.
  3. “The furcula is present (in Gallus) by HH 33 as a bilateral pair of condensations that have not fused in the midline” — This sounds like clavicles to me.
  4. “The pectoral apparatus of basal ornithodirans (falsely including pterosaurs) is incompletely known, and it remains unclear which if any mid-ventral dermal element was present.” That list is shown above. 
  5. Although previously interpreted as clavicles (in Psittacosaurus and Massospondylus), the identity of these elements is herein considered equivocal.” — no reason given, no phylogenetic path proposed.
  6. Vickaryous and Hall suggest the incorporation of the clavicle and interclavicle into the sternal complex of pterosaurs is restricted to one juvenile, and “has yet to be demonstrated in other taxa.” – This is what scientists and PterosaurHeresies readers would immediately call ‘recognizing a presence in the literature, but minimizing its impact on the present study and avoiding any effort at finding out what the situation actually is in other pterosaurs.’ I have also looked at the sternal complex of many pterosaurs and have observed that the interclavicle and clavicles are incorporated into the sternal complex of ALL pterosaurs and a few outgroup taxa, in which you can see the process happening. At present, and after reading Vicaryous and Hall 2006, I see no reason to homologize the interclavicle and the furcula.

References
Bryant HN and Russell  AP 1993. The occurrence of of clavicles within Dinosauria: Implications for the homology of the avian furcula and the utility of negative evidence. Journal of Vertebrate Paleontology 13(2):171–184.
Nesbitt S, Turner AH, Spaulding M and Norell MA 2009. The Theropod Furcula. Journal of Morphology 270(7):856–879.
Wild R 1993. A juvenile specimen of Eudimorphodon ranzii Zambelli (Reptilia, Pterosauria) from the upper Triassic (Norian) of Bergamo. Rivisita Museo Civico di Scienze Naturali “E. Caffi” Bergamo 16: 95–120.
Vicaryous MK and Hall BK 2006. Homology of the reptilian coracoid and a reappraisal of the evolution and development of the amniote pectoral apparatus Journal of Experimental Zoology (Molecular and Developenmtal Evolution) 314B 196-207.

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