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.

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.

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.

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.

Old views on dull-witted semi-aquatic dinosaurs correct after all…

A new study suggests dinosaurs were aquatic... too heavy to move on land.

For more than half a century, dinosaurs were depicted as slow-moving creatures living in swampy environments. Giants such as the 35 tons Brachiosaurus were deemed too heavy on land to be able to support their own weight and the prominent view for most of the 20th century was that they were amphibious animals that spent most of their time half-submerged in water, grazing on soft aquatic plants. Dinosaurs were also thought to be incredibly stupid in view of their very small braincase. For instance, the plated Stegosaurus had a brain the size of a walnut which is indeed minuscule for an animal reaching 9 m in length. This view on dinosaurs drastically changed starting in the 1960s (the so-called “dinosaur renaissance”) when new discoveries such as the numerous fossils of the duck-billed dinosaur Maiasaura found at “Egg Mountain” in Montana, and encompassing individuals of all ages from hatchlings to adults, purportedly showed that dinosaurs raised their youngs and thus had intelligence matching those of modern mammals and birds. Today, dinosaurs are ubiquitously portrayed as highly active fully terrestrial creatures capable of a wide range of social behavior such as pack hunting and parental care.  

The old view of dinosaurs as dull-witted amphibious beasts has been totally abandoned and these animals are now often seen as one of the prime evolutionary successes rather than as a failed nature experiment doomed for extinction. However, in recent years, scattered discoveries indicated that at least some dinosaurs were aquatic: in the 1970s, a new specimen of the little theropod Compsognathus  found in southern France was described as having webbed feet indicative of a semi-aquatic lifestyle.  In 2010, the analysis of the oxygen isotope ratios in Spinosaurus teeth showed that it was close to those of crocodiles and aquatic turtles, proving that the giant sailed theropod spent lots of time in water. In 2011, a new ceratopsian dinosaur, Koreaceratops, was described as having a tail adapted for swimming. Now, a new study published in the April 2014 issue of the Journal of the Australian Society of Vertebrate Palaeontology, shows that the aquatic lifestyle of dinosaurs was actually widespread. The study applied the same isotopic analysis technique used for Spinosaurus, on the teeth of some 20 species of sauropods (the long-necked, long-tailed giants such as Diplodocus and Brachiosaurus) and ornithopods (including the duck-billed dinosaurs such as Parasaurolophus and  Edmontosaurus) from the Late Jurassic and Late Cretaceous of North America. The results show for all teeth a level of oxygen isotopes compatible with life in water. “This technique has until now only been applied to Spinosaurus because a fish-eating diet was long suspected for this theropod due to its crocodile-like snout. Nobody thought to check on dinosaurs such as Brachiosaurus or Edmontosaurus as the general consensus was that these were fully terrestrial” says lead author Dr. Avril Zierste from the Palaeontology department of the University of Sao Paulo. “The results came quite as a shock because the current views on these ancient creatures will have to take a 180 degree turn”, she adds. Many observed dinosaurian features that were left unexplained as land animals, make perfect sense if they were water dwelling gentle giants. For instance, the flattened tail of duck-billed dinosaurs rigidified by ossified tendons, were superbly adapted for swimming. Moreover the often complex nasal apparatus of many of these same duck-billed dinosaurs were probably used as snorkels to breathe while underwater, as initially hypothesized well before the “dinosaur renaissance". Sauropod bones are well known to show high degree of pneumaticity, which are seen as evidence of the presence of air sacs. This purpose of this becomes clear if these animals were aquatic as air sacs will help with buoyancy and act as floating devices. It is probably not a coincidence that fossils of dinosaurs around the world were predominantly found in regions that were very close to water and often among remains of marine or freshwater animals such as fish and crocodiles.

Old depiction of Brachiosaurus depicted as aquatic animals by Czech painter Z. Burian (1905-1981). New study suggests early paleontologists were right from the beginning about the lifestyle of dinosaurs.

Is it possible that the current view of terrestrial dinosaurs with high metabolism portrayed after  the “dinosaur renaissance” was more based on wishful thinking than hard evidences? The eminent scholar Josh Mothorn certainly thinks so: “We were so keen to believe that no group of animals could have dominated the earth for 165 millions years without sharing at least some of the qualities of our kind, the mammals, such as warm-bloodedness and intelligence, that any circumstances tending to indicate that they were something else than slow moving idiotic monsters, were branded as evidence of complex behavioral habit. The fact is that movies such as “Jurassic Park” would never have been successful if the heroes were confronted not by incredibly fast and skillful killing machines but by clumsy sluggish creatures that could be easily fooled and outsmarted”.  “The dinosaur renaissance is dead” concludes Art Kerbebrok after hearing the conclusions of the study “it is high time that we stop imagining them as nothing more than stupid giants too heavy to support their own weight. People were quick to dismiss the obvious: their brain to body size ratios were among the smallest in any tetrapods. Dinosaurs were slow, gigantic and idiotic. Mammals have outwitted them, that’s why dinosaurs are extinct”.  In light of the new study by Dr. Zierste and co-workers, it seems that the old prevalent view of dinosaur as slow-witted and placid semi-aquatic giants was correct after all.

Reference:

Avril Z., Peter S. G., Robert T. B., Ines L. P., Lambert J. B., Stewart M.T., Fernando E. N., Oviedo T. F., Oscar J., Lewis C. C. (April 1, 2014) “Oxygen isotope analysis and brain-body ratio measurements of 20 species of Sauropoda and Ornithopoda as evidence for semi-aquatic lifestyle and low I.Q. for the clade Dinosauria” J. of Australian Society of Vertebrate Palaeontology, 90., 1-12.