Wednesday, April 30, 2014

The enigmatic jawless fish Jamoytius kerwoodi

A reconstruction of Jamoytius kerwoodi
The exact affinities of the jawless fish Jamoytius have been at the heart of scientific controversies and heated debate during most of the second part of the 20th century (Ritchie, 1984). The first fossils of this early vertebrate were discovered in 1914 near Lesmahagow, Lanarkshire, central Scotland in beds dating from the Late Silurian period, but awaited more than 20 years before a formal description was published (White, 1946). The species Jamoytius kerwoodi is based on two specimens and was first considered to be a primitive naked fish-like chordate possibly ancestral to the cephalochordate Amphioxus (also called Lancelet). Later studies however reinterpreted the “carbonized muscle remains” described by White as being weakly mineralized scales, similar to those seen in another grade of jawless fish, the Anaspida, and Jamoytius was naturally placed among them. Its branchial apparatus similar to those of a lamprey made it at one point, a possible ancestor of the petromyzontiformes (the group that include the modern lampreys). These branchial openings (gill slits) numbered from 10 to 15 or more and were therefore more numerous than in modern lampreys (seven). The discovery of a few more fossils did little to help settle the debate and details such as the shape and position of the fins remain greatly hypothetical. It appears to have had a dorsal fin, an anal fin and a hypocercal caudal fin although the latter is rather inferred from related species such as Endeiolepis than genuinely observed as preserved in the fossils. The presence of paired fins along the body has never been very conclusive.

It had relatively large eyes and a single nostril. The mouth was circular without teeth. Some authors proposed that Jamoytius represented the larval stage of an ostracoderm or even of a cephalochordate (Wickstead, 1969). This is rather unlikely as the animal has an elongated body of 15 to up to 35 cm, making it one of the largest jawless fish from the Silurian period. The most recent study of this animal (Sansom et al., 2010) using a combination of topological reconstruction, comparative anatomy, elemental mapping and phylogenetic analysis, concluded that Jamoytius and its relatives were definite vertebrates and stem gnathostomes rather than ancestors of lampreys or relatives of the Anaspida.

The absence of teeth indicates that it was either a filter feeder or a detritus feeder. An interesting theory linked to its supposed affinity with lampreys make it a possible suctorial feeder (Ritchie, 1968). In this theory, it is implied that the numerous circular perforations observed in the enigmatic organism Dictyocaris were made by Jamoytius. However there is nothing to back up that claim besides the matching size of the holes with the mouth of Jamoytius. Jamoytius fossils were found alongside numerous remains of another agnathan, Thelodus scoticus, fossils of the problematic taxon Ainiktozoon loganense (most recent study makes it an arthropod), as well as of many arthropods and a few molluscs in what constituted a marine environment. Anyhow, to date, the enigmatic Jamoytius is with Thelodus, still the oldest vertebrate known from the European continent.

References:

Ritchie, A. (1960). A new interpretation of Jamoytius kerwoodi White. Nature, 188(4751), 647–649.
Ritchie, A. (1968). New evidence on Jamoytius kerwoodi White, an important ostracoderm from the Silurian of Lanarkshire, Scotland. Palaeontology, 11(1), 21–39.
Ritchie, a. (1984). Conflicting interpretations of the Silurian agnathan, Jamoytius. Scottish Journal of Geology, 20(2), 249–256.
Sansom, R. S., Freedman, K., Gabbott, S. E., Aldridge, R. J., & Purnell, M. a. (2010). Taphonomy and affinity of an enigmatic Silurian vertebrate, Jamoytius kerwoodi White. Palaeontology, 53(6), 1393–1409.
Wickstead, J. (1969). Some further comments on Jamoytius kerwoodi White. Zoological Journal of the Linnean Society, 48(August), 421–422.

3 comments:

  1. "Anapsida"

    Sometimes I wonder if taxonomists are deliberately cruel. It's Anaspida -- Anapsida is an amniote group (or possibly a grade).

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    1. Very nice summary -- was not aware that the latest opinion had it as a stem-gnathostome.

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    2. Oops... Can't believe I fell for that one. No wonder I was thinking of turtles (which are not even anapsids anymore). Thanks for catching :)

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