Monday, September 1, 2014

Meet the Chroniosuchians



Chroniosaurus dongusensis measured about 30 cm in length.
Chroniosuchians form a very obscure group of small, superficially croc-like four-legged animals that lived during the Late Permian period and survived until the Middle and perhaps Late Triassic periods. Most species are from the Late Permian (Late Tatarian regional stage) of Russia, but they are also known from China, Germany and Kyrgyzstan. They were aquatic or semi-aquatic fish-eating predators characterized by a distinct row of interconnected “butterfly-shaped” bony plates or scutes (osteoderms) covering their back and part of their tail, one for each vertebra. They are traditionally subdivided into two families, the more primitive Chroniosuchidae from the Late Permian with one representative in the Triassic and the Bystrowianidae from the Late Permian to the Middle Triassic. The exact affinities of the Chroniosuchians are quite uncertain. They are usually placed among non-amniote  reptiliomorphs but more recent studies make them “stem tetrapods”, so not reptiles and not quite amphibians but something a bit more primitive. The eel-like embolomeres appear to have been their closest relatives.

Madygenerpeton pustulatus from Kyrgyzstan.
Among the Chroniosuchids, Chroniosaurus dongusensis Tverdochlebova, 1972 from the Late Permian of the Orenburg province of Russia is probably the best known species, with fossils from many individuals including one near complete articulated specimen. Chroniosuchus paradoxus Vjuschkov, 1957 also from the Orenburg province but a bit younger, differs in the taller shape of its skull. Madygenerpeton pustulatus Schoch et al., 2010 from the Middle or Late Triassic of Kyrgyzstan appears to be the basalmost member of the group despite its late occurrence. The type specimen is a nearly complete skull showing a broad snout quite different from the two previous species. Jarilinus mirabilis (Vjuschkov, 1957) from the Novgorod province and Uralerpeton  tverdochlebovae Golubev, 1998 from the Vladimir Province were quite large species for the group, the reconstructed skull of the former measuring 20 cm and of the latter some 55 cm. The others three species, Chroniosaurus levis Golubev, 1908, Chroniosuchus lichaveri (Riabinin, 1962) and Suchonica vladimiri Golubev, 1999, all from the Vologda Province of Russia, are only known from fragmentary remains.

Chroniosuchus paradoxus.
The other family, the Bystrowianids, are much less known, their fossil remains consisting mainly on isolated armor scutes. Late permian species include Bystrowiana permira Vjuschkov, 1957 from the Vladimir Province of Russia, Bystrowiana sinica Young, 1979, Dromotectum largum Liu et al., 2014 and Jinyuanitectum flatum Liu et al., 2014, the last three from the Henan Province of China. Early Triassic species are Axitectum vjushkovi Shishkin & Novikov, 1992 from the Novgorod province, Axitectum georgi Novikov & Shishkin, 2000 from the Kirov province, Dromotectum spinosum Novikov & Shishkin, 1996 from the Orenburg Province. Middle Triassic forms are Synesuchus muravjevi Novikov & Shishkin, 2000 from the Komi Republic and Bystrowiella schumanni Witzmann et al., 2008 from Germany.

References:

Buchwitz, M., Foth, C., Kogan, I., & Voigt, S. (2012). On the use of osteoderm features in a phylogenetic approach on the internal relationships of the Chroniosuchia (Tetrapoda: Reptiliomorpha). Palaeontology, 55(3), 623–640.
Golubev, V. K. (1998). Narrow-armored Chroniosuchians from the Late Permian of Eastern Europe. Paleontological Journal, 32(3), 278–287.
Golubev, V. (1998). Revision of the Late Permian chroniosuchians (Amphibia, Anthracosauromorpha) from Eastern Europe. Paleontological Journal, 32(4), 390–401.
Golubev, V. K. (1999). A New Narrow-Armored Chroniosuchian from the Upper Permian of Eastern Europe. Paleontological Journal, 33(2), 166–173.
Klembara, J., Clack, J. a., & Čerňanský, A. (2010). The anatomy of palate of Chroniosaurus dongusensis (Chroniosuchia, Chroniosuchidae) from the Upper Permian of Russia. Palaeontology, 53(5), 1147–1153.
Liu, J., Xu, L., Jia, S.-H., Pu, H.-Y., & Liu, X.-L. (2014). The Jiyuan tetrapod fauna of the Upper Permian of China — 2 . stratigraphy , taxonomical review , and correlation. Vertebrata PalAsiatica, 52(3), 328–339.
Schoch, R., Voigt, S., & Buchwitz, M. (2010). A chroniosuchid from the Triassic of Kyrgyzstan and analysis of chroniosuchian relationships. Zoological Journal of the Linnean Society, 160, 515–530.
Witzmann, F., Schoch, R. R., & Maisch, M. W. (2008). A relict basal tetrapod from Germany: first evidence of a Triassic chroniosuchian outside Russia. Die Naturwissenschaften, 95(1), 67–72.

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.