Castorocauda

Taxobox
name = "Castorocauda"
fossil_range = Middle Jurassic

---

image_width = 250px
regnum = Animalia
phylum = Chordata
subphylum = Vertebrata
unranked_classis = Amniota
classis = Synapsida
unranked_ordo = Mammaliaformes
ordo = Docodonta
familia = Docodontidae
genus = "Castorocauda"
species = "C. lutrasimilis"
binomial = "Castorocauda lutrasimilis"
binomial_authority = Ji "et al.", 2006

"Castorocauda lutrasimilis" (also known as the "Jurassic Beaver") is the name given to a small, semi-aquatic relative of mammals living in the middle Jurassic period, 164 million years ago, in lakebed sediments of the Daohugou Beds (possibly a member of the Jiulongshan formation) of Inner Mongolia. It was highly specialized, with adaptations evolved convergently with those of modern semi-aquatic mammals such as beavers, otters, and the platypus. It was larger than any known contemporaneous mammal.

Classification

"Castorocauda" is not considered to be a mammal by most authorities (Ji et al., 2006). The technical, cladistic, definition of a mammal is the group containing the most recent common ancestor of all living mammals (including monotremes, placentals, and marsupials) and its descendants. "Castorocauda" is a member of the order Docodonta, which is a wholly extinct group of Mammaliaformes, or proto-mammals. It has no known modern descendants.

An important goal of paleontologists is to track the origin and evolution of certain characteristics. Hard anatomy characters such as teeth and bones preserve well in the fossil record and are the main source of information about how fossil animals are related to their modern counterparts. Soft anatomy features such as internal organs do not preserve readily.

A fossil discovery was made in 2004 in the fossil-rich beds of Liaoning province, China; it was reported in the journal "Science" by an international team led by Qiang Ji of Nanjing University. The fossil of "Castorocauda" was so well preserved that an important feature of its soft anatomy— hair— was preserved. Hair is present in all modern mammals and was therefore assumed, under principles of maximum parsimony, to have been present in all fossil true mammals. The presence of hair in "Castorocauda" indicates that hair was not only present in mammals, but also in their closest relatives, the docodonts. In fact, the hair appears to have been a very advanced dense pelage including guard hairs and underfur.

The tiny auditory ossicles of the middle ear and associated areas were also well preserved in this "Castorocauda" fossil. Features of these bones confirms the evolutionary position of docodonts as less closely related to true mammals than "Hadrocodium", but more closely related to mammals than other mammaliaforms such as "Morganucodon" and "Sinoconodon".

Among docodonts, "Castorocauda" appears to have been related to "Krusatodon" and "Simpsonodon", both European animals. This may be evidence that Europe and Asia underwent a faunal interchange in the Middle Jurassic. The two continents would later be separated by the Turgai Strait.

Adaptation to water

The name "Castorocauda lutrasimilis" is derived from the Latin "castor"- meaning "beaver", -"cauda" meaning "tail", "lutra" meaning "otter", and -"similis" meaning "similar to". The tail was broad with scales interspersed with hairs that grew less frequent toward the tip. Overall it was very similar to the tails of modern beavers and was presumably used for locomotion in water in a similar fashion. The caudal vertebrae were flattened dorso-ventrally and similar overall to those in a beaver or otter. Fossilized impressions of some webbing is also present between the toes.

Features of the limbs suggested that it may have been adapted for digging. The forelimbs are robust, with enlarged olecranon and other processes associated with strong muscle attachment. The limbs are similar to the modern platypus, an animal that both digs and swims. "Castorocauda", "Haldanodon" and perhaps other docodonts were fossorial. These early specializations were also present in the unique early true mammal "Fruitafossor".

Docodonts in general have distinctive teeth, and the teeth of "Castorocauda" have the distinguishing features of the group. The teeth of "Castorocauda" are different in many ways from all other docodonts, presumably due to a difference in diet. Most docodonts had teeth specialized for an omnivorous diet. The teeth of "Castorocauda" suggest that the animal was a piscivore, feeding on fish and small invertebrates. The first two molars had cusps in a straight row, eliminating the grinding function suggesting that they were strictly for gripping and not for chewing. This feature of three cusps in a row is similar to the ancestral condition in mammal relatives (as seen in triconodonts), but is almost certainly a derived character in "Castorocauda". These first molars were also recurved in a manner designed to hold slippery prey once grasped. These teeth are very similar to the teeth seen in mesonychids, an extinct group of semi-aquatic carnivorous ungulates, and resemble, to a lesser degree, the teeth of seals.

The complete dental formula was not recoverable, but the lower jaw contained 4 incisors, 1 canine, 5 premolars, and 6 molars.

The animal probably weighed about 500-800 grams (1 pound to nearly 2 pounds) and grew to at least 42.5 cm (17 inches) in length. This makes it the largest mammaliaform (including true mammals) of the Jurassic. The previous record holder was "Sinoconodon" which was thought to weigh up to 500 g.

Fossil evidence

The fossil was from the Jiulongshan Formation of the Inner Mongolia region of China. Fossils of pterosaurs, lissamphibians, coelurosaurian dinosaurs, and numerous invertebrates have also been unearthed in the same formation.

It was discovered and described by Qiang Ji and Chong-Xi Yuan of the Chinese Academy of Geological Sciences in Beijing and Zhe-Xi Luo and Alan Tabrum of the Carnegie Museum of Natural History.

Importance of discovery

The discovery of "Castorocauda lutrasimilis" is the first sign that a close relative of mammals adapted to water before dinosaurs lost dominance 65 million years ago, pushing back the estimated date for mammal relatives adapted to a semi-aquatic lifestyle by 110 million years. Based on fossils known at present, the mammal line would not see another semi-aquatic form evolve until the Eocene. Because few fossilized remains had been found, it was previously thought that, until the KT boundary, all mammals were tiny, ground-dwelling or tree-dwelling, nocturnal animals akin to shrews, hedgehogs, treeshrews, or tenrecs. This notion has now been falsified by the armadillo-like "Fruitafossor", the dinosaur-eating "Repenomamus", the flying squirrel-like "Volaticotherium" and now the otter-like "Castorocauda".

References

*Ji, Q., Z.-X. Luo, C.-X. Yuan, A. R. Tabrum. February 24, 2006. "A swimming mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals". "Science", 311:5764 pp.1123-1127.

External links

* [http://www.carnegiemnh.org/news/06-jan-mar/022306caud.htm Carnegie Museum's Press release with images]
* [http://livescience.com/animalworld/060223_aquatic_mammal.html Live Science article with artist's impression]
* [http://www.timesonline.co.uk/article/0,,25689-2055852,00.html Times Online article]
* [http://www.cnn.com/2006/TECH/science/02/23/jurassic.beaver.ap/index.html CNN article]
* [http://abcnews.go.com/Technology/story?id=1648586&page=1 ABC News article]
* [http://www.fossilmuseum.net/UD%20desktop/UD_destop_postings/Paleobiology/Castorocauda.htm Fossil Museum: "Castorocauda lutrasimilis"]