Two late Scottish relatives of Jamoytius

Cornovichthys blaauweni.

The now flooded Achanarras fish beds in Caithness, Northern Scotland, has yielded an abundant collection of fossil fish belonging to some 15 genera dating from the Eifelian stage (about 390 MYA) of the Middle Devonian period. These include the arthrodire (a group of heavily armored jawed fish) Coccosteus cuspidatus, the Acanthodians (“spiny sharks”) Diplacanthus, Mesacanthus and Cheiracanthus, the enigmatic Palaeospondylus gunni, the early Actinopterygian (ray-finned fish) Cheirolepis treilli and the Sarcopterygians (lobed-finned fish) Osteolepis macrolepidotus and Dipterus valenciennesi. The agnathans (jawless fish) are a rarity there and consist of only two species, Cornovichthys blaaweni and Achanarella trewini, which are believed to be closely related to Jamoytius kerwoodi from the Silurian, and placed in the clade Jamoytiiforms. The limestone of Achanarras quarry has been deposited in what appear to have been a deep freshwater lake environment.

Cornovichthys blaauweni is known from a single carbonaceous impression of a complete specimen measuring a little over 10 cm in length. The body is long and slender with a hypocercal tail and a quite large anal fin. There is no evidence of any other fin, nor of any scale in the specimen. Eyes appear to be situated near the top surface of the head. Overall, Cornovichthys looks a lot like Euphanerops and like its Canadian counterpart has a long series of branchial openings running from the head to the anal fin, numbering to around 15 on each side of the body.

Achanarella trewini.

Achanarella trewini is known from many specimens that were found in large numbers on single slabs of rock. Individuals of Achanarella range in size from 2 cm to 9 cm in length. Like Cornovichthys, it has a hypocercal tail and a large anal fin, no apparent scale and a large number of branchial openings on each side of the body, 13 or more, but the body is much thinner and elongated and the head is extremely small. Just like Jamoytius and Euphanerops, Cornovichthys and Achanarella were probably feeding on micro-organisms and detritus through their jawless mouth.

Reference:

Newman, M., & Trewin, N. (2001). A new jawless vertebrate from the Middle Devonian of Scotland. Palaeontology, 44, 43–51.

Newman, M. (2002). A new naked jawless vertebrate from the Middle Devonian of Scotland. Palaeontology, 45(5), 933–941.

The Canadian cousins of Jamoytius

Euphanerops longaevus

The rich Late Devonian fish fauna of Miguasha, in Eastern Quebec, Canada (Escuminac Formation) includes some peculiar jawless fish closely related to the Silurian scottish species Jamoytius kerwoodi. They have a strongly hypocercal tail with a relatively large anal fin. The body is elongated and have a series of long and narrow weakly mineralized scales on the flanks. The eyes are relatively large and the mouth is a circular opening situated at the bottom of the head. Like most if not all primitive fish, they lack paired fins. The branchial openings were numerous numbering 30 or so (lampreys have only 7 of these gill pouches) aligned from the head to the anal region, and therefore stretching over a very long portion of the body. This peculiar arrangement is thought to be an adaptation to a poorly oxygenated water. The first species to be described is Euphanerops longaevus by the British paleontologist Sir Arthur Smith Woodward in 1900. This strange animal was however originally described upside down, with the anal fin as a dorsal one and an epicercal tail. Euphanerops measured about 10 cm in length. The second species is Endeiolepis aneri described by the swedish paleontologist Erik Stensiö in 1939. It is very similar to Euphanerops and it was suggested that the two represent the same animal, with Euphanerops being the juvenile form. In that case, the name Euphanerops has priority and Endeiolepis would be a junior synonym. Legendrelepis parenti described by M. Arsenault and P. Janvier, 1991 is considered to be a junior synonym as well, any noted differences such as the alleged presence of a dorsal fin are now viewed as artifacts of preservation.

References:

Janvier, P., Desbiens, S., Willett, J. a, & Arsenault, M. (2006). Lamprey-like gills in a gnathostome-related Devonian jawless vertebrate. Nature, 440(7088), 1183–5. 

Janvier, P., & Arsenault, M. (2007). The anatomy of Euphanerops longaevus Woodward, 1900, an anaspid-like jawless vertebrate from the Upper Devonian of Miguasha, Quebec, Canada. Geodiversitas, 29(1), 143–216.

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.