''Castorocauda'' is an extinct, semi-aquatic, superficially
otter-like
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of
docodont mammaliaforms with one species, ''C. lutrasimilis''. It is part of the
Yanliao Biota
The Yanliao Biota is the name given to an assembly of fossils preserved in northeastern China from the Middle to Late Jurassic.Xu, X., Zhou, Z., Sullivan, C. and Wang, Y., 2017. The Yanliao Biota: a trove of exceptionally preserved Middle-Late Jur ...
, found in the
Daohugou Beds
The Haifanggou Formation (), also known as the Jiulongshan Formation (), is a fossil-bearing rock deposit located near Daohugou () village of Ningcheng County, in Inner Mongolia, northeastern China.
The formation consists of coarse conglomerat ...
of
Inner Mongolia
Inner Mongolia, officially the Inner Mongolia Autonomous Region, is an autonomous region of the People's Republic of China. Its border includes most of the length of China's border with the country of Mongolia. Inner Mongolia also accounts for a ...
, China dating to the
Middle to
Late Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
. It was part of an explosive
Middle Jurassic radiation of Mammaliaformes moving into diverse habitats and
niches. Its discovery in 2006, along with the discovery of other unusual mammaliaforms, disproves the previous hypothesis of Mammaliaformes remaining evolutionarily stagnant until the extinction of the non-avian
dinosaurs.
Weighing an estimated , ''Castorocauda'' is the largest known
Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
mammaliaform. It is the earliest known mammaliaform with aquatic adaptations or a fur
pelt
Fur is a thick growth of hair that covers the skin of mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an insulating blanket ...
. It was also adapted for digging, and its teeth are similar to those of seals and
Eocene
The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...
whales, collectively suggesting it behaved similarly to the modern-day
platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...
and river otters and ate primarily fish. It lived in a wet, seasonal, cool
temperate
In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...
environment – which possibly had an average temperature not exceeding – alongside salamanders, pterosaurs, birdlike dinosaurs, and other mammaliaforms.
Discovery and etymology
The
holotype specimen
A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...
,
JZMP04117, was discovered in the
Daohugou Beds
The Haifanggou Formation (), also known as the Jiulongshan Formation (), is a fossil-bearing rock deposit located near Daohugou () village of Ningcheng County, in Inner Mongolia, northeastern China.
The formation consists of coarse conglomerat ...
of the
Jiulongshan Formation in the
Inner Mongolia
Inner Mongolia, officially the Inner Mongolia Autonomous Region, is an autonomous region of the People's Republic of China. Its border includes most of the length of China's border with the country of Mongolia. Inner Mongolia also accounts for a ...
region of China, which dates to about 159–164
million years ago
The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds.
Usage
Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago) ...
(mya) in the
Middle to
Late Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
.
[ It comprises a partial skeleton including an incomplete skull but well-preserved lower jaws, most of the ribs, the limbs (save for the right hind leg), the pelvis and the tail. The remains are so well preserved that there are elements of its soft anatomy and hair.][
The ]genus name
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
''Castorocauda'' derives from Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''Castor'' " beaver" and ''cauda'' "tail", in reference to its presumed beaver-like tail. The species name
In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...
''lutrasimilis'' derives from Latin ''lutra'' " otter" and ''similis'' "similar", because some aspects of its teeth and vertebra
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
e are similar to modern otters.[
]
Description
''Castorocauda'' was the largest of known docodonts. The preserved length from head to tail is , but in life it was much larger. Based on the dimensions of the platypus, the lower weight limit was estimated to be in life, and the upper , making it the largest known Jurassic mammaliaform, surpassing the previous record of for ''Sinoconodon
''Sinoconodon'' is an extinct genus of mammaliamorphs that appears in the fossil record of the Lufeng Formation of China in the Sinemurian stage of the Early Jurassic period, about 193 million years ago. While sharing many plesiomorphic tra ...
''.[
The recurved teeth of ''Castorocauda'' are more are similar in shape to those of ]mesonychia
Mesonychia ("middle claws") is an extinct taxon of small- to large-sized carnivorous ungulates related to artiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely ...
ns, Eocene
The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...
whales, and seals than other docodonts. The first two molars have cusps in a straight row, and interlocked during biting. This feature is similar to the ancestral condition in Mammaliaformes (such as in triconodont
Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the p ...
s) but is a derived character (it was specially evolved instead of inherited) in ''Castorocauda''.[ The lower jaw contained 4 ]incisor
Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...
s, 1 canine, 5 premolars and 6 molars.[
The forelimbs of ''Castorocauda'' are very similar to those of the modern platypus: the humerus widens towards the ]elbow
The elbow is the region between the arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and the me ...
; the forearm bones have hypertrophied (large) epicondyles (where the joint attaches); the radial
Radial is a geometric term of location which may refer to:
Mathematics and Direction
* Vector (geometric), a line
* Radius, adjective form of
* Radial distance, a directional coordinate in a polar coordinate system
* Radial set
* A bearing f ...
and ulna
The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
l joints are widely separated; the ulna has a massive olecranon
The olecranon (, ), is a large, thick, curved bony eminence of the ulna, a long bone in the forearm that projects behind the elbow. It forms the most pointed portion of the elbow and is opposite to the cubital fossa or elbow pit. The olecranon ...
(where it attaches to the elbow); the wrist bones are block-like; and the finger bones are robust. Docodontans were likely burrowing creatures and had a sprawling gait, and ''Castorocauda'' may have also used its arms for rowing, similar to the platypus. There are traces of soft tissue between the toes, suggesting webbed
''Webbed'' is a 2D adventure puzzle platform game developed and published by Australian, Brisbane-based studio Sbug Games. A physics-based game set in a fantasy version of Queensland, players control a peacock spider whose goal is to rescue ...
hind feet. It likely also had claw
A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or tarsus ...
s, and the holotype shows a spur
A spur is a metal tool designed to be worn in pairs on the heels of riding boots for the purpose of directing a horse or other animal to move forward or laterally while riding. It is usually used to refine the riding aids (commands) and to ba ...
on the hind ankle, which, in male platypuses, is venomous.[
''Castorocauda'' likely had 14 ]thoracic
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
, 7 lumbar, 3 sacral
Sacral may refer to:
*Sacred
Sacred describes something that is dedicated or set apart for the service or worship of a deity; is considered worthy of spiritual respect or devotion; or inspires awe or reverence among believers. The property ...
and 25 tail vertebrae. Like some mammals, it had plated ribs, and the ribs extended into the lumbar vertebrae. Plating occurred on the proximal margins (the part of the rib closest to the vertebra), and, in ''Castorocauda'', they may have served to increase the insertion area (the part of a muscle which moves while contracting) of the iliocostalis
Iliocostalis muscle is the muscle immediately lateral to the longissimus that is the nearest to the furrow that separates the epaxial muscles from the hypaxial. It lies very deep to the fleshy portion of the serratus posterior muscle. It latera ...
muscle on the back, which would interlock nearby ribs and better support the torso of the animal.[ Plated ribs are present in ]arboreal
Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose nu ...
(tree-dwelling) and fossorial (burrowing) xenarthra
Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. ...
ns (sloth
Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their l ...
s, anteaters, armadillos and relatives). The tail vertebrae are flattened dorsoventrally (shortened vertically and widened more horizontally); and each centrum
(Latin for ''center'') may refer to:
Places In Greenland
* Nuuk Centrum, a district of Nuuk, Greenland
* Centrum Lake, Greenland In the Netherlands
* Amsterdam-Centrum, the inner-most borough of Amsterdam, Netherlands
* Rotterdam Centrum, a borou ...
has two pairs of transverse processes (which jut out diagonally from the centrum) on the headward side and another on the tailward side, making the centrum appear somewhat like the letter H from the top-view looking down. This tail anatomy is similar to beavers and otters, which use their tails for paddling and propulsion.[
Fur was preserved on the holotype, and it is the earliest known ]pelt
Fur is a thick growth of hair that covers the skin of mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an insulating blanket ...
;[ this showed that fur, with its many uses including heat retention and as a tactile sense, was an ancestral trait of mammals. Mammals preserved with fur from the Chinese ]Yixian Formation
The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its exq ...
show little hair on the tail, whereas the fur outline preserved on the ''Castorocauda'' tail was 50% wider than the pelvis. The first quarter is covered by guard hair
Guard hair or overhair is the outer layer of hair of most mammals, which overlay the fur. Guard hairs are long and coarse and protect the rest of the pelage (fur) from abrasion and frequently from moisture. They are visible on the surface of the fu ...
s, the middle half by scales and little hair cover and the last quarter by scales with some guard hair. Beavers have a very similar tail.[ Evidence of fur and assumed heightened tactile senses indicate it had a well-developed neocortex, a portion of the brain unique to mammals which, among other things, controls ]sensory perception
Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...
.
Taxonomy
''Castorocauda'' is a member of the order Docodonta, an extinct group of mammaliaforms. Mammaliaformes includes mammal-like creatures and the crown
A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...
mammals (all descendants, living or extinct, of the last common ancestor of all living mammals). Docodonts are not crown mammals. When ''Castorocauda'' was first described in 2006, it was thought to be most closely related to the European '' Krusatodon'' and '' Simpsonodon''.[ In a 2010 review of docodonts, Docodonta was split into ]Docodontidae
Docodonta is an order of extinct mammaliaforms that lived during the Mesozoic, from the Middle Jurassic to Early Cretaceous. They are distinguished from other early mammaliaforms by their relatively complex molar teeth, from which the order ge ...
, Simpsonodontidae and Tegotheriidae, with ''Castorocauda'' considered ''incertae sedis
' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...
'' with indeterminate affinities. Simpsonodontidae is now considered to be paraphyletic and thus invalid, and ''Castorocauda'' appears to have been most closely related to '' Dsungarodon'',[ which came from the ]Junggar Basin
The Junggar Basin () is one of the largest sedimentary basins in Northwest China. It is located in Xinjiang, and enclosed by the Tarbagatai Mountains of Kazakhstan in the northwest, the Altai Mountains of Mongolia in the northeast, and the Heave ...
of China and probably ate plants and soft invertebrates.
''Castorocauda'' is part of a Middle Jurassic mammaliaform diversification event, wherein mammaliaforms radiated into a wide array of niches and evolved several modern traits, such as more modern mammalian teeth and middle ear
The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear).
The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...
bones.[ It was previously thought that mammals were small and ground-dwelling until the Cretaceous–Paleogene boundary (K–Pg boundary) when dinosaurs went extinct. The discovery of ''Castorocauda'',] and evidence for an explosive diversification in the Middle Jurassic – such as the appearance of eutriconodontans, multituberculate
Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, a ...
s, australosphenida
The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. They are thought to have acquired their tribosphenic molars independently from those of Tribosphenid ...
ns, metatheria
Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as wel ...
ns and eutheria
Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials.
Eutherians are distinguished from noneutherians by various phenotypic tra ...
ns, among others – disproves this notion. This may have been caused by the breakup of Pangaea
Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
, which started in the Early to Middle Jurassic and diversified habitats and niches, or modern traits that had been slowly accumulating since mammaliaforms evolved until reaching a critical point which allowed for a massive expansion into different habitats.
Paleoecology
''Castorocauda'' is the earliest known aquatic mammaliaform, pushing back the first appearance of mammaliaform aquatic adaptations by over 100 million years.[ The teeth interlocked while biting, suggesting that they were strictly used for gripping; the recurved molars were likely used to hold slippery prey; and the teeth shapes are convergent with seals and Eocene whales, suggesting a similar ecological standing. Based on these, its adaptations to swimming and digging and its large size, ''Castorocauda'' was probably comparable to the modern day ]platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...
, river otters and similar semi-aquatic mammals in ecology
Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...
and fed primarily on fish (piscivory
A piscivore () is a carnivorous animal that eats primarily fish. The name ''piscivore'' is derived . Piscivore is equivalent to the Greek-derived word ichthyophage, both of which mean "fish eater". Fish were the diet of early tetrapod evolut ...
).[
The Daohugou Beds also include several ]salamander
Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
s, numerous pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
species (of which many likely were piscivorous),[ several insects, the ]clam shrimp
Clam shrimp are a group of bivalved branchiopod crustaceans that resemble the unrelated bivalved molluscs. They are extant and also known from the fossil record, from at least the Devonian period and perhaps before. They were originally classifi ...
'' Euestheria''[ and some birdlike dinosaurs. No fish are known from specifically the Daohugou Beds, but the related Linglongta locality contains undetermined ptycholepiformes. Other mammals include the flying-squirrel-like '']Volaticotherium
''Volaticotherium antiquum'' (meaning "ancient gliding beast") is an extinct, gliding, insectivorous mammal that lived in Asia during the Jurassic period, around 164 mya. It is the only member of the genus ''Volaticotherium''.
The discovery of ...
'', the burrowing ''Pseudotribos
''Pseudotribos'' ("false chewing") is an extinct genus of mammal that lived in Northern China during the Middle Jurassic some , possibly more closely related to monotremes than to theria (placental and marsupial mammals), although other studies i ...
'', the oldest known eutherian ''Juramaia
''Juramaia'' is an extinct genus of very basal eutherian mammal known from the Late Jurassic ( Oxfordian stage) deposits of western Liaoning, China. It is a small shrew-like mammal with a body length of approximately 70–100 mm, making it ...
''. the rat-like '' Megaconus'' and the gliding '' Arboroharamiya''. The plant life of the Tiaojishan Formation was dominated by cycadeoids (mainly '' Nilssonia'' and '' Ctenis''), leptosporangiate fern
The Polypodiidae, commonly called leptosporangiate ferns, formerly Leptosporangiatae, are one of four subclasses of ferns, and the largest of these, being the largest group of living ferns, including some 11,000 species worldwide. The group has ...
s and ginkgophytes and has pollen remains predominantly from pteridophytes and gymnosperm
The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμν ...
s, which indicate a cool temperate
In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
and wet climate with distinct wet and dry seasons,[ possibly with an annual temperature of below .]
See also
*''Haldanodon
''Haldanodon'' is an extinct docodont mammaliaform which lived in the Upper Jurassic (Kimmeridgian, about 145 million years ago). Its fossil remains have been found in Portugal, in the well-known fossil locality of Guimarota, which is in the Al ...
''
*'' Borealestes''
References
Further reading
*
External links
Carnegie Museum's Press release
{{Taxonbar, from=Q131336
Docodonts
Jurassic animals of Asia
Fossil taxa described in 2006
Taxa named by Qiang Ji (paleontologist)
Taxa named by Zhe-Xi Luo
Taxa named by Chong-Xi Yuan
Taxa named by Alan R. Tabrum
Prehistoric cynodont genera