Sunday, July 20, 2014

Dinosaurs of the British Isles: the Book

For those of you who were wondering why I never completed my series on British dinosaurs on paleoexhibit, stopping midway through the theropods, here is the reason: the material has been used as a starting point for a book. Written in collaboration with Doncaster Museum very own paleontologist and good friend, Dean Lomax, the book has just been published by Siri Scientific Press and is available for purchase on Amazon (link from the ad on the left). Prefaced by Dr. Paul Barrett of the Natural History Museum, “Dinosaurs of the British isles” consists of over 400 pages describing in details some 100 species of dinosaurs unearthed for over two centuries in the United Kingdom. It features extensive amount of photographs of fossil remains taken by Dean as he visited all the major paleontology collections from England, Scotland and Wales, skeletal reconstructions from Scott Hartman, Jaime Headden and Greg Paul, as well as life reconstructions by talented artist James McKay, and by myself. Highly recommended to anybody interested in fossils in general, and British dinosaurs in particular.

Reference: D. R. Lomax & N. Tamura, 2014. “Dinosaurs of the British Isles”, Siri Scientific Press, 414 pages, ISBN 13: 9780957453050, ISBN 10: 0957453051

Wednesday, May 21, 2014

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.

Sunday, May 4, 2014

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.

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.

Sunday, April 20, 2014

Jawless armored fish from the Ordovician: the Astraspids and Eriptychiids

My reconstruction of Astraspis desiderata.

The Astraspids and Eriptychiids form other groups of jawless armored fish restricted to the Ordovician. They are the earliest known definite vertebrates from North America. Like the Arandaspids, they apparently all went extinct at the end of the period. Unlike the Arandaspids, their head shield carapace is made of hundreds of very small and separate bony pieces called tesserae. They have polygonal shape surmounted by tubercles of various morphologies. Their ornamentation and shape are different depending on their position in the carapace, so that a tesserae from the dorsal plate can be distinguished from one from the ventral plate. Two genera are known, Astraspis and Eriptychius, both originally described from the same location, the Late Ordovician (~ 455 MYA) Harding Sandstone, near Cañon City in Colorado, United States, and in the same 1892 paper authored by Charles D. Walcott, the discoverer of the famous Burgess Shale. It is still unclear how these two genera are related and they were often placed in two separate families, Astraspidae and Eriptychiidae or even orders, Astraspida and Eriptychiida. The microstructure of Eriptychius’s tesserae placed them closer to the Heterostraci than Astraspis.

Astraspis desiderata is the better known species with at least three mostly complete and articulated specimens. It had 8 branchial openings on its sides with well developed eyes in the front. The tail is made of large rhomboid scales. Astraspis desiderata measured about 20 cm in length. A seemingly  larger species, Pycnaspis splendens has been described in 1958 by Swedish paleontologist Tor Ørvig from the Harding Sandstone of Bighorn Mountains in Wyoming, United States but this one is now considered to be synonymous with Astraspis desiderata. Isolated tesserae attributed to Astraspis have been found throughout North America, in Colorado, Wyoming, South Dakota, Ontario, Quebec and Oklahoma

Eriptychius americanus was only known from isolated and highly ornamented tesserae. The situation changed dramatically in 1967 with the discovery of an articulated specimen consisting of the front part of the dorsal shield (Denison, 1967). A second species, Eriptychius orvigi, honoring Ørvig and from the Bighorn Mountains of Wyoming, with seemingly thicker tesserae, was erected in the same paper, but is now considered to be synonymous with E. americanus.

Just like the Arandaspids, the Astraspids and Eriptychiids did not have any paired fins and would have been rather poor swimmers. They were probably living at the bottom of the sea floor in a sublittoral environment.

References:
Sansom, I. J., Smith, M. P., Smith, M. M., & Turner, P. (1997). Astraspis-the anatomy and histology of an Ordovician fish. Palaeontology, 40(3), 625–643.
Elliott, D. K. (1987). A Reassessment of Astraspis desiderata, the Oldest North American Vertebrate. Science (New York, N.Y.), 237(4811), 190–2.
Bryant, W. (1936). A study of the oldest known vertebrates, Astraspis and Eriptychius. Proceedings of the American Philosophical Society, 76(4), 409–427.
Denison, R. (1967). Ordovician vertebrates from western United States. Fieldiana: Geology, 16(6), 131–192.

Sunday, April 13, 2014

Jawless armored fish from the Ordovician: the Arandaspids

Jawless armored fish from the Ordovician: the Arandaspids

My reconstruction of Sacabambaspis janvieri.
The fossil record of fish during the Ordovician, the period that follows the Cambrian around 485 MYA, is quite poor and consists of just a little more than a handful of named taxa. One of the prominent groups of that time appears to be the Arandaspids. They were jawless (a condition shared by all other vertebrates in these ancient seas) and characterized by a head covered with a bony shield consisting of a flattish dorsal plate, a rounded ventral plate, and a few other smaller plates. Arandaspids were quite primitive looking with two eyes and two nostrils in the front, a series of branchial openings, each protected by bony platelets, on the side between the dorsal and ventral plates. The back portion of the animal was protected by strips of bony armor arranged in chevrons. They had a caudal fin, but no paired fin, making them not particularly good swimmers. They probably lived on the seafloor feeding on microorganisms or organic detritus sucked in through their jawless mouth. All Arandaspids were marine.

My reconstruction of Arandaspis prionotolepis.
The type species of the group is Arandaspis prionotolepis from the shallow marine deposits of the Stairway Sandstone in the Northern Territory, Australia, and dating from the Earliest Middle Ordovician. This 10-15 cm long fish, originally described in 1977 (Ritchie & Gilbert-Tomlinson, 1977) is known from several specimens, some quite complete. The other relatively well known species is Sacabambaspis janvieri from the Anazaldo Formation of Bolivia, which was discovered among a fauna composed almost exclusively of lingulid brachiopods, an indication that it lived near the littoral in a well oxygenated area. Initially described from three bone fragments in 1986 (Gagnier & Blieck, 1986), new fossils were later found including a complete articulated specimen (Pradel et al., 2007) that preserved the uniquely shaped hypocercal tail (the end tip of the vertebral column bends downward supporting the bottom lobe of the tail). The Anzaldo Formation was originally believed to be of Early Upper Ordovician age, but it may actually have been older making Sacabambaspis quite contemporary with Arandaspis (Gagnier et al., 1996). Sacabambaspis was a bit larger than its Australian counterpart, reaching a length of 25 cm.

Articulated fossil specimen of Sacabambaspis janvieri. From User:Ghedoghedo, Wikipedia commons
Another fish, Andinaspis suarezorum from the Capinota Formation of Bolivia, known from a single poorly preserved fragment, was once thought to be Early Middle Ordovician and classified as a possible Arandaspid, but there are now doubts on its actual age, which turned out to be in all probability Devonian (Gagnier et al., 1996). Also from Bolivia, but from the Pircancha Formation of Early Ordovician age, comes what seems to a be ventral shield of a possible large Arandaspid christened Pircanchaspis rinconensis (Erdtmann et al., 2000) This is the earliest record of a fish from South America. From Australia, Porophoraspis crenulata from the same location and age than Arandaspis was described in the same paper than the latter,  but is much less known as only a single external mould of a small plate has been recovered. This one is also a possible Arandaspid.

Arandaspids belong to one of the two major groups of armored jawless fish that would dominate the first part of the Paleozoic era: the Heterostraci or Heterostracomorphs, the other group being the Cephalaspids. Arandaspids apparently did not last longer than the Ordovician, being replaced by far more efficient forms in the Silurian.

References:

Erdtmann, B., Weber, B., Schultze, H.-P., & Egenhoff, S. (2000). A possible agnathan plate from the Lower Arenig (Lower Ordovician) of South Bolivia. Journal of Vertebrate Paleontology, 20(2), 394–399.

Gagnier, P., Blieck, A., & G., R. S. (1986). First Ordovician vertebrate from South America. Geobios, 19(5), 629–634.

Gagnier, P., Blieck, A., Emig, C., Sempere, T., Vachard, D., & Vanguestaine, M. (1996). New paleontological and geological data on the Ordovician and Silurian of Bolivia. Journal of South American Earth Sciences, 9(5/6), 329–347.

Pradel, A., Sansom, I. J., Gagnier, P.-Y., Cespedes, R., & Janvier, P. (2007). The tail of the Ordovician fish Sacabambaspis. Biology Letters, 3(1), 73–76.

Ritchie, A., & Gilbert-Tomlinson, J. (1977). First Ordovician vertebrates from the southern hemisphere. Alcheringa, 1(4), 351–368.

Sunday, April 6, 2014

Early vertebrates: the Myllokunmingiidae

My reconstruction of the early vertebrate Haikouichthys ercaicunensis, based on Zhang & Hou, 2004.
Early vertebrates: the Myllokunmingiidae

Molecular data of extant fauna places the divergence of vertebrates (animals with a backbone, including mammals, birds, reptiles, amphibians and fish) from their closest relatives, the cephalochordates (Amphioxus) as far back as 751 MYA (Hedges, 2001) during the Cryogenian period of the proterozoic. This is well before the Ediacaran biota (575 MYA) and the so-called Cambrian explosion (542 MYA). However, it was recently shown that the molecular clock ran some five times faster during the Cambrian than during any other period that followed (Lee, 2013). A more conservative and reasonable estimate would therefore make the vertebrates appeared at the very end of the Proterozoic or during the Lower Cambrian along many other phyla. What does the fossil record says? For a while, the earliest undisputed vertebrate fossil remains consisted of some isolated dermal bones dating from the Early Ordovician (480 MYA) of central Australia and belonging to a group of jawless fish called Arandaspida. This situation changed quite a bit in 1999 with the discovery and description of two fossils from the famous Chengjiang biota of the Maotianshan Shale in the Yunnan Province of China, dating from the middle of the Lower Cambrian (525-520 MYA). Two species were erected, Myllokunmingia fengjiaoa Shu et al., 1999 and Haikouichthys ercaicunensis Luo et al., 1999, regrouped into the family Myllokunmingiidae. A third species, Zhongjianichthys rostratus Shu, 2003 has been added to the list four years later.

Haikouichthys (“Haikou fish”) is by far the best known of the three. Originally based on a single incomplete specimen, it is today known from more than 500 specimens from the same fossil locality near Haikou, in the Kunming prefecture of Yunnan. Measuring about 2.5 cm in length, Haikouichthys has an elongated fish-like body with a single dorsal, ventral and caudal fin, as shown from a remarkably well-preserved specimen (Zhang & Hou, 2004). It is not easy to interpret faint impressions within the fossils but structures and internal organs such as vertebrae, paired eyes, guts, heart, and possibly a nostril, an olfactory organ have been identified. Haikouichthys had a number of gill pouches and series of W shape myomeres (muscle blocks that are typical in fish).  The presence of a mouth can only be inferred as it is not clearly visible in the fossils. Haikouichthys was certainly an active swimmer but probably not a good one because of the lack of paired fins. Haikouichthys and the other Myllokunmingiids appear to be the most primitive agnathans (jawless fish). Phylogenetic analysis indicates that this is a stem vertebrate more primitive than lampreys and any other known jawless fish.

Myllokunmingia (“Kunming fish”) is known from a single 2.8 cm long specimen. It is usually seen as being a bit larger and bulkier than Haikouichthys. However, a new fossil (Hou et al., 2002) showing a combination of characters found in Haikouichthys and Myllokunmingia, may indicate that the two constitute in fact a single animal (in that case, the name Myllokunmingia would have precedence over Haikouichthys) and any observed differences may rather reflect preservation bias. This view is however not universally recognized. Zhongjianichthys is a problematic animal that has been classified as a Myllokunmingiid, but not enough is known about it for this attribution to be certain. Another Maotianshan Shale animal, Haikouella lanceolata Chen, Huang & Li, 1999, known from more than 300 specimens, is often considered as another possible stem vertebrate. However it looks so similar to the contemporaneous Yunnanozoon lividum, a possible hemichordate or stem chordate, that this attribution is somewhat unlikely. Most specimen of Haikouella measured 2.5 to 3 cm in length with some individuals reaching 4 cm.

Reference:

Chen, J., Huang, D., & Li, C. (1999). An early Cambrian craniate-like chordate. Nature, 402(December), 518–522.

Hedges, S. (2001). Molecular evidence for the early history of living vertebrates. In Major Events in Early Vertebrate Evolution, 119–134.

Lee, M. S. Y., Soubrier, J., & Edgecombe, G. D. (2013). Rates of phenotypic and genomic evolution during the Cambrian explosion. Current Biology, 23(19), 1889–95.

Shu, D., Luo, H., Morris, S., Zhang, X., & Hu, S. (1999). Lower Cambrian vertebrates from south China. Nature, 402(November), 42–46.

Shu, D., Morris, S., Han, J., & Zhang, Z. (2003). Head and backbone of the Early Cambrian vertebrate Haikouichthys. Nature, 421(January), 526–529.

Xian-guang, H., Aldridge, R. J., Siveter, D. J., Siveter, D. J., & Xiang-hong, F. (2002). New evidence on the anatomy and phylogeny of the earliest vertebrates. Proceedings. Biological Sciences / The Royal Society, 269(1503), 1865–9.

Zhang, X.-G., & Hou, X.-G. (2004). Evidence for a single median fin-fold and tail in the Lower Cambrian vertebrate, Haikouichthys ercaicunensis. Journal of Evolutionary Biology, 17(5), 1162–6.