Questions on the Branchial Structure of Thelodonts

[Mainefossils]

So, I have been doing more research on thelodont morphology, and I have a few questions regarding the branchial structure, and was hoping for some other people's understanding on this subject. 

 

Regarding the branchial structure, what is the most current research? I have read that the species Pezopallichthys ritchiei, Cometicercus talimaaae, Drepanolepis maersae, Sphenonectris turnerae, Turinia pagei, Furcacauda heintzae, and Furcacauda fredholmae, have shown themselves to be among the best preserved articulated thelodonts, with pretty well preserved branchial bars and openings. The picture below is of an excellent illustration of one of the species, from: Donoghue, P. C. J., Smith, M. P. (2001) The anatomy of Turinia pagei (Powrie), and the phylogenetic status of the Thelodonti. Transactions of the Royal Society of Edinburgh: Earth Sciences, 92. It shows a T. pagei, and the anatomy of this fish. 

 

                                                                  

 

However, it is still debated as to the exact nature of this structure. I have read that the general consensus on the matter is that the transverse ridges seen on these specimens are from the internal cartilaginous branchial skeleton. Each branchial opening is thought to have been covered by a flap of scales. Some specimens preserve black stains in the these openings, that are supposedly the remnants of the cartilaginous skeleton. I would appreciate anyone's insight on this matter, and their views on the structure of the gills of thelodonts - as well as any new research that I am unaware of. 

 

Another picture from the same paper, showing the holotype specimen under three different lighting conditions: 

 

           

 

And two more from a separate paper that details the morphology of Furcacaudiformes from Northern Canada: Wilson, M. V. H., Cladwell, M. W. (1998) The Furcacaudiformes: A New Order of Jawless Vertebrates with Thelodont Scales, Based on Articulated Silurian and Devonian Fossils from Northern Canada. Journal of Vertebrate Paleontology, 18(1). The first one is a reconstruction of F. fredholmae, the second is of P. ritchiei; an excellently preserved thelodont, with well preserved gill openings and branchial bars.  

 

                            

 

                                                                 

 

Thanks everyone for reading this (lengthy) post! I look forward to seeing your thoughts. 

 

References: 

 

Donoghue, P. C. J., Smith, M. P. (2001) The anatomy of Turinia pagei (Powrie), and the phylogenetic status of the Thelodonti. Transactions of the Royal Society of Edinburgh: Earth Sciences, 92. 

 

Marss, T., Turner, S., Karatajute-Talimaa, V. (2007) "Agnatha" II, Handbook of Paleoichthyology, Volume 1B. (Schultze, H.-P., ed.). Pfeil, Munich, 143 pp. 

 

Stetson, H. C. (1931) Studies on the Morphology of the Heterostraci. The Journal of Geology, 39(2). - details the morphology and structure of the thelodonts Thelodus scotica (debated whether branchial bars are present), Thelodus planus, and Thelodus taiti (now Loganellia taiti). 

 

Turner, S. (1982) A new articulated thelodont (Agnatha) from the early Devonian of Britain. Paleontology, 25(4). 

http://www.ewyaslacy.org.uk/docs/nw_nwt_2002.pdf - details the similarities of T. pagei with the new specie N. milesi, discusses the branchial apparatus and other organs. 

 

Wilson, M. V. H., Cladwell, M. W. (1998) The Furcacaudiformes: A New Order of Jawless Vertebrates with Thelodont Scales, Based on Articulated Silurian and Devonian Fossils from Northern Canada. Journal of Vertebrate Paleontology, 18(1).

 

Wilson, M. V. H., Marss, T. (2012) Anatomy of the Silurian thelodont Phlebolepis elegans Pander. Estonian Journal of Earth Sciences, 61(4).

https://www.researchgate.net/publication/258421627_Anatomy_of_the_Silurian_thelodont_Phlebolepis_elegans_Pander - details the anatomy of P. elegans, also discusses the apparent absence of any trace of branchial bars or opening. 

 

[Mainefossils]

Probably should've tagged you, @jdp. 

[jdp]

With these sorts of specimens, what you see is what you get. I don't know if any more recent work has been done on thelodont gill arch morphology but my assumption would be that this is relatively rarely preserved so it is rare that fossils are found that move the discussion forward in a meaningful way.