Drepanolepis, an Early Devonian ‘fork-tail’ fish, enters the LRT

I dreaded this one, fearing its weirdness…
but after a little DGS coloring (Fig. 1) came to realize this tiny furcacaudiforme (Early Devonian fork-tail fish?) was just like another fish already in the large reptile tree (LRT, 1677+ taxa), only shorter and narrower. After analysis the two nested together.

Drepanolepis maerssae (Wilson and Caldwell 1993, 1998; Early Devonian; 2cm in length) is a traditional ‘thelodont’ and a member of the Furcacaudiformes (forked tails). In the LRT Drepanolepis is derived from Birkenia (Fig. 2), but with a taller, shorter, more angelfish-like body. They both have a ventral mouth and a hypocercal tail, somewhat elaborated in Drepanolepis with several posterior processes. The gill atrium remains quite large and the nasal extends from the orbit down to the oral cavity, which remains like that of a lancelet, without jaws. Without jaws there is no premaxilla, maxilla quadrate, articular, angular and dentary.

Birkenia is not traditionally considered a thelodont.
All other traditional thelodonts, like Thelodus and Loganiella, have a low, wide morphology. Thelodus has a ventral oral cavity and nests with osteostracans and sturgeons in the LRT. Loganiella has a wide terminal mouth and nests with whale sharks and mantas in the LRT. Drepanolepis and other furcacaudiformes do not nest with these traditional thelodonts in the LRT and should no longer be considered thelodonts.

Birkenia and furcacaudiformes 
bridge the gap between lancelets and gnathostomes. The have the body and bones of a basal fish, but retain a lancelet-like oral cavity that cannot be called a proper mouth. That comes later.

By the Early Devonian
fish had evolved to such an extent that some had lobefins (the sarcopterygians) and others had exoskeletons (the placoderms).

Let’s talk about the traditional ‘terminal mouth’ of Furcacauda.
Wilson and Caldwell 1998 restored a terminal mouth on Furcacauda (Fig. 4) but did so by guessing. The skull is missing from the specimen (Fig. 4). No other furcacaudiformes have a terminal mouth (Figs. 5,6). All other furcacaudiformes (Figs. 1,5,6) described and figured by Wilson and Caldwell have an overlooked ventral oral cavity, like that of Birkenia. In addition I rotated one image, that of Sphenonectris (Fig. 6), to bring the dorsal side to the top for proper orientation.

It has been 22 years since Wilson and Caldwell 1998 was published.
Perhaps in the meantime someone else has noticed these issues. If so, let me know and I will promote that citation.

As in lancelets
the oral cavity of Birkenia and furcacaudiformes can never close and is surrounded by oral cirri that work as sand filters.

As in Birkenia
precursors to tetrapod skull bones can be found in furcacaudiformes for the first time phylogenetically. Even so, the short, narrow Furcacaudiformes displayed here today are members of a terminal clade with no descendants later than the Devonian. Closer sisters to Birkenia evolved to become basal vertebrates (fish and tetrapods).

We looked at Birkenia
and the origin of dermal facial bones from splintery scales earlier here. using similar DGS methods.

References
Traquair RH 1898. Report on fossils fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897: 72-76.
Wilson MVH and Caldwell MW 1993. New Silurian and Devonian fork-tailed ‘thelodonts’ are jawless vertebrates with stomachs and deep bodies. Nature. 361 (6411): 442–444.
Wilson MVH and Caldwell MW 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): 10-29.

wiki/Thelodus
wiki/Birkenia
wiki/Drepanolepis
wiki/Furcacaudiformes