Drepanolepis ancestors

One of the wierdest fish ever, EVER, EVER, was
Drepanolepis (Figs. 1, 2). Wikipedia says very little about this taxon, other than to say it is “a thelodont.”

Figure 1. Drepanolepis, traced from Wilson and Caldwell 1998, has a ventral oral cavity and nests with Birkenia in the LRT.
Figure 1. Drepanolepis, traced from Wilson and Caldwell 1998, has a ventral oral cavity and nests with Birkenia in the LRT.

By contrast,
in the large reptile tree (LRT, 1839+ taxa; subset Fig. 3) Drepanolepis is not related to Early Silurian Thelodus (a sturgeon ancestor), but to Jaymoytius, Birkenia and Euphanerops (Fig. 2). So, Drepanolepis is not a thelodont, despite having similar distinctive scales.

Turns out this taxon is best studied
by direct examination AND by comparative anatomy. Until now, outgroup taxa were either not known or mistaken.

We first looked
at the fork-tailed fish, Drepanolepis, and re-interpreted the location of the circular lancelet-like oral cavity (not quite a mouth) a year ago here. At that time Drepanolepis nested only with Birkenia (Fig. 2). Now two other taxa join this clade (Fig. 3).

Jamoytius – Early Silurian
Birkenia – Middle Silurian
Euphanerops – Late Devonian (late survivor from an earlier radiation)
Drepanolepis – Early Devonian

Figure 2. Two drepanolepiids and ancestors back to Jamoytius, shown to scale and actual size if viewed on a 72 dpi monitor.

One key to understanding this clade
is to remember that the lancelet-like oral cavity opened ventrally, rather than anteriorly as first imagined by Wilson and Caldwell (1993, 1998). That makes them all bottom feeders on microscopic prey.

Another key to understanding this clade
is to remember that drepanolepid ancestors had a hypocercal tail with fringes on top (Fig. 1). The top-most fringe of a taxon like Euphanerops (Fig. 1) continued to grow (phylogenetically) until it matched the lower fleshy portion of the tail in Furcacauda (Fig. 2), the portion that always and forever included a notochord.

Figure 3. Subset of the LRT focusing on basal chordates including Drepanolepis (above) and a thelodont, below. The former is tall and narrow. The latter is low and flat and basal to later vertebrate taxa. All these taxa precede the Gnathostomata (Chondrosteus is the last common ancestor).

All these taxa have
a sharp little rostrum, a pair of eyeballs, a precursor anal fin, patches that will someday become skull bones and a ventral gill basket with a series of tiny circular gill openings.

All these taxa lack
paired nares, paired pectoral fins, paired pelvic fins, dorsal fins, teeth and jaws.

Drepanolepis maerssae
(Wilson and Caldwell 1993, 1998; Early Devonian; 2cm in length) is a traditional thelodont and a member of the Furcacaudiformes (forked tails). Drepanolepis is derived from Jamoytius, Birkenia and Euphanerops (Fig. 2), but with a smaller, taller, shorter, angelfish-like body. These taxa have a ventral ‘mouth’ and a hypocercal tail, somewhat elaborated in Euphanerops and more so in Drepanolepis. The gill atrium remains quite large and the nasal extends from the orbit down to the oral cavity.

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
wiki/Furcacauda

wiki/Thelodonti

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