Gill chambers in basal chordates and vertebrates, pt. 3

Earlier and even earlier we looked at the large gill chambers
in basal chordates and basal armored fishes. Today we’ll look at some even larger gill chambers in basal unarmored fishes, some of them living today.

We start with Loganellia
(Fig. 1), a small, wide, Silurian unarmored traditional ‘thelodont’, basal to Rhincodon (Figs. 2, 6), today’s largest fish, the whale shark.

Figure 1. Loganellia, from the Silurian, had a large, wide gill chamber inside a low skull.

Figure 1. Loganellia, from the Silurian, had a large, wide gill chamber inside a low skull.

Both are/were planktonic open water feeders
with enormous gill chambers for ingesting swarms of tiny prey and sifting them from the seawater exiting out the several lateral gill slits.

Figure 2. The whale shark, Rhincodon, has an enormous gill chamber for capturing planktonic prey.

Figure 2. The whale shark, Rhincodon, has an enormous gill chamber for capturing planktonic prey.

Early Devonian (410mya) Turinia pagei
(Powrie 1870; Figs. 3, 4) was re-described by Donoghue and Smith 2001. It “possessed a branchial system composed of eight pairs of gills, a buccal/nasohypophyseal region lined with minute denticles, comparable to buccopharyngeal denticles of sharks, and possessed a stomach which is preserved by sediment infill in the holotype specimen.” Little can be said of the skeleton, but the tooth plate in the palate described above is visible along with supporting elements that could be terminal jaws. Like Rhincodon, Turinia devoted its entire torso to a huge gill chamber. Note the anterior extension of the pectoral fins to the corner of the mouth.

Table 1 in Donoghue and Smith
lists twelve previous authors who speculated on the affinities of Turinia.

  1. ray-like – 3 authors, including Powrie 1870
  2. thelodont or Thelodus –  6 authors
  3. neotenic osteostracan – 1 author
  4. heterostracan or a derived shark ancestor of a heterostracan – 2 authors
Figure 2. Turinia is a basal ray, possibly ancestral to mantas or cow nose rays.

Figure 3. Turinia, in ventral view,  is a basal ray, possibly ancestral to mantas or cow nose rays.

Figure 4. Turinia holotype from Donoghue and Smith 2001, plus their diagram, plus DGS tracings. Note the anterior extension of the pectoral fin, as in manta rays.

Figure 4. Turinia holotype from Donoghue and Smith 2001, plus their diagram, plus DGS tracings. Note the anterior extension of the pectoral fin, as in manta rays. This specimen is exposed in ventral view with the skin overlying the stomach (yellow) missing. 

Turinia could be an ancestor
to today’s manta ray (Manta, Fig. 5) which differs little other than greatly increasing the size and extent of its giant pectoral fins while reducing its tail. Donoghue and Smith made no mention of Rhincodon or Manta in their paper, so this is a novel hypothesis of interrelationships.

Figure 3. The gill chamber and digestive track of Manta shown in ventral view.

Figure 5. The gill chamber and digestive track of Manta shown in ventral view.

Rather
Donoghue and Smith restricted their taxon list in phylogenetic analysis to 33 extinct jawless taxa (including Tunicata) plus one taxon labeled ‘jawed vertebrates”. They report, “Phylogenetic analysis resolves T. pagei and the Galeaspida as sister-taxa, comprising a sister-group to the Osteostraci plus jawed vertebrates.”

Figure 4. Subset of the LRT with the addition of several jawless taxa.

Figure 6. Subset of the LRT with the addition of several jawless taxa.

Based on the present phylogenetic analysis,
presented at the large reptile tree (LRT; 1611+ taxa; subset Fig. 6), the Rhincodon + Manta clade includes Longanellia, long considered to be a thelodont. However, these taxa are a few nodes distant from Thelodus. Sturgeons and the chimaera + shark clade intervene. Turinia does not have enough pertinent traits to enter the LRT. Perhaps future analyses that include the present taxon list will someday clarify these issues.


References
Donoghue PCJ and Smith MP 2001. The anatomy of Turinia pagei (Powrie), and the phylogenetic status of the Thelodonti. Transactions of the Royal Society of Edinburgh: Earth Sciences 92:15–37. PDF
Powrie J 1870. On the earliest known vestiges of vertebrate life; being a description of the fish remains of the Old Red Sandstone rocks of Forfarshire. Edinburgh Geological Society Transactions 1: 284–301.

Turinia pagei Powrie 1870 Fossilworks

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