Castorocauda lutrasimilis
Encyclopedia
Castorocauda was a genus
of small, semi-aquatic relative of mammal
s living in the mid Jurassic
period, around 154 million years ago, found in lakebed sediments of the Daohugou Beds
of Inner Mongolia
. It was highly specialized, with adaptations evolved convergently
with those of modern semi-aquatic mammals such as beaver
s, otter
s, and the platypus
.
-definition, as belonging to the group containing the most recent common ancestor of all living mammals (including monotreme
s, placentals, and marsupial
s) 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. The crown group concept is however not universally accepted. Kielan-Jaworowska et al. (2002), for example, continued to include the mammaliaformes in Mammalia.
An important goal of paleontologist
s 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 hair
s 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.
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 invertebrate
s. The first two molar
s 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 mesonychid
s, an extinct group of semi-aquatic carnivorous
ungulate
s, and resemble, to a lesser degree, the teeth of seals
.
The complete dental formula was not recoverable, but the lower jaw contained 4 incisor
s, 1 canine, 5 premolar
s, 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.
of the Inner Mongolia
region of China. Fossils of pterosaur
s, lissamphibians, coelurosauria
n dinosaur
s, and numerous invertebrate
s 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
.
. Because few fossilized remains had been found, it was previously thought that, until the K–T boundary
, all mammals were tiny, ground-dwelling or tree-dwelling, nocturnal animal
s akin to shrew
s, hedgehog
s, treeshrew
s, 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.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of small, semi-aquatic relative of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s living in the mid Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
period, around 154 million years ago, found in lakebed sediments of the Daohugou Beds
Daohugou Beds
The Daohugou Beds are a series of fossil-bearing rock deposits located in northeastern China, with the type locality around Daohugou village of Ningcheng County south of Chifeng, Inner Mongolia, but extending into southwest Liaoning. The rocks are grey, finely bedded, lacustrine, sandy mudstones...
of Inner Mongolia
Inner Mongolia
Inner Mongolia is an autonomous region of the People's Republic of China, located in the northern region of the country. Inner Mongolia shares an international border with the countries of Mongolia and the Russian Federation...
. It was highly specialized, with adaptations evolved convergently
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
with those of modern semi-aquatic mammals such as beaver
Beaver
The beaver is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, North American Beaver and Eurasian Beaver . Beavers are known for building dams, canals, and lodges . They are the second-largest rodent in the world...
s, otter
Otter
The Otters are twelve species of semi-aquatic mammals which feed on fish and shellfish, and also other invertebrates, amphibians, birds and small mammals....
s, and the platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
.
Classification
Castorocauda is not considered to be a mammal by the crown groupCrown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
-definition, as belonging to the group containing the most recent common ancestor of all living mammals (including monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s, placentals, and marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s) and its descendants. Castorocauda is a member of the order Docodonta
Docodonta
Docodonta is an order of extinct proto-mammals that lived during the mid- to late-Mesozoic era. Their most distinguishing physical features were their relatively sophisticated set of molars, from which the order gets its name. In the fossil record, Docodonta is represented primarily by isolated...
, which is a wholly extinct group of Mammaliaformes
Mammaliaformes
Mammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...
, or proto-mammals. It has no known modern descendants. The crown group concept is however not universally accepted. Kielan-Jaworowska et al. (2002), for example, continued to include the mammaliaformes in Mammalia.
An important goal of paleontologist
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
s is to track the origin and evolution of certain characteristics. Hard anatomy
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
characters such as teeth and bones preserve well in the fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
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
Nanjing University
Nanjing University , or Nanking University, is one of the oldest and most prestigious institutions of higher learning in China...
. The fossil of Castorocauda was so well preserved that an important feature of its soft anatomy— hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
— was preserved. Hair is present in all modern mammals and was therefore assumed, under principles of maximum parsimony
Maximum parsimony
Parsimony is a non-parametric statistical method commonly used in computational phylogenetics for estimating phylogenies. Under parsimony, the preferred phylogenetic tree is the tree that requires the least evolutionary change to explain some observed data....
, 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 hair
Guard hair
Guard hairs are the longest, coarsest hairs in a mammal's coat, forming the topcoat . They taper to a point and protect the undercoat from the elements. They are often water repellent and stick out above the rest of the coat...
s and underfur.
The tiny auditory ossicles of the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
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
Hadrocodium
Hadrocodium wui is an extinct basal mammal species that lived during the Lower Jurassic in what is now the Yunnan province of China...
, but more closely related to mammals than other mammaliaforms such as Morganucodon
Morganucodon
Morganucodon is an early mammalian genus which lived during the Late Triassic. It first appeared about 205 million years ago. This has also been identified with Eozostrodon. Unlike many other early mammals, Morganucodon is well represented by abundant and well preserved, though in the vast...
and Sinoconodon
Sinoconodon
Sinoconodon rigneyi is an ancient proto-mammal that appears in the fossil record in the late Triassic period, about 208 million years ago. Although the animal seems more related to Morganucodon than anything else, it differed substantially from other Mammaliaformes in its dental and growth habits...
.
Among docodonts, Castorocauda appears to have been related to Krusatodon and Simpsonodon, both European animals. This may be evidence that Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
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 LatinLatin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
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
Fossorial
A fossorial organism is one that is adapted to digging and life underground such as the badger, the naked mole rat, and the mole salamanders Ambystomatidae...
. These early specializations were also present in the unique early true mammal Fruitafossor
Fruitafossor
Fruitafossor was a termite-eating mammal endemic to North America during the Late Jurassic epoch , existing for approximately ....
.
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
Omnivore
Omnivores are species that eat both plants and animals as their primary food source...
diet. The teeth of Castorocauda suggest that the animal was a piscivore
Piscivore
A piscivore is a carnivorous animal which eats primarily fish. Piscivory was the diet of early tetrapods , insectivory came next, then in time reptiles added herbivory....
, feeding on fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and small invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s. The first two molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
s 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 mesonychid
Mesonychid
Mesonychia are an extinct order of medium to large-sized carnivorous mammals that were closely related to artiodactyls and to cetaceans...
s, an extinct group of semi-aquatic carnivorous
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
ungulate
Ungulate
Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...
s, and resemble, to a lesser degree, the teeth of seals
Pinniped
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...
.
The complete dental formula was not recoverable, but the lower jaw contained 4 incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s, 1 canine, 5 premolar
Premolar
The premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...
s, 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
Sinoconodon
Sinoconodon rigneyi is an ancient proto-mammal that appears in the fossil record in the late Triassic period, about 208 million years ago. Although the animal seems more related to Morganucodon than anything else, it differed substantially from other Mammaliaformes in its dental and growth habits...
which was thought to weigh up to 500 g.
Fossil evidence
The fossil was from the Daohugou BedsDaohugou Beds
The Daohugou Beds are a series of fossil-bearing rock deposits located in northeastern China, with the type locality around Daohugou village of Ningcheng County south of Chifeng, Inner Mongolia, but extending into southwest Liaoning. The rocks are grey, finely bedded, lacustrine, sandy mudstones...
of the Inner Mongolia
Inner Mongolia
Inner Mongolia is an autonomous region of the People's Republic of China, located in the northern region of the country. Inner Mongolia shares an international border with the countries of Mongolia and the Russian Federation...
region of China. Fossils of pterosaur
Pterosaur
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...
s, lissamphibians, coelurosauria
Coelurosauria
Coelurosauria is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. In the past, it was used to refer to all small theropods, although this classification has been abolished...
n dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s, and numerous invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s 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
Beijing
Beijing , also known as Peking , is the capital of the People's Republic of China and one of the most populous cities in the world, with a population of 19,612,368 as of 2010. The city is the country's political, cultural, and educational center, and home to the headquarters for most of China's...
and Zhe-Xi Luo and Alan Tabrum of the Carnegie Museum of Natural History
Carnegie Museum of Natural History
Carnegie Museum of Natural History, located at 4400 Forbes Avenue in the Oakland neighborhood of Pittsburgh, Pennsylvania, USA, was founded by the Pittsburgh-based industrialist Andrew Carnegie in 1896...
.
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 EoceneEocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
. Because few fossilized remains had been found, it was previously thought that, until the K–T boundary
K–T boundary
The K–T boundary is a geological signature, usually a thin band, dated to 65.5 ± 0.3 Ma ago. K is the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period...
, all mammals were tiny, ground-dwelling or tree-dwelling, nocturnal animal
Nocturnal animal
Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. The common adjective is "nocturnal"....
s akin to shrew
Shrew
A shrew or shrew mouse is a small molelike mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different families or orders.Although its external appearance is generally that of...
s, hedgehog
Hedgehog
A hedgehog is any of the spiny mammals of the subfamily Erinaceinae and the order Erinaceomorpha. There are 17 species of hedgehog in five genera, found through parts of Europe, Asia, Africa, and New Zealand . There are no hedgehogs native to Australia, and no living species native to the Americas...
s, treeshrew
Treeshrew
The treeshrews are small mammals native to the tropical forests of Southeast Asia. They make up the families Tupaiidae, the treeshrews, and Ptilocercidae, the pen-tailed treeshrews, and the entire order Scandentia. There are 20 species in 5 genera...
s, or tenrecs. This notion has now been falsified by the armadillo
Armadillo
Armadillos are New World placental mammals, known for having a leathery armor shell. Dasypodidae is the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths. The word armadillo is Spanish for "little armored one"...
-like Fruitafossor
Fruitafossor
Fruitafossor was a termite-eating mammal endemic to North America during the Late Jurassic epoch , existing for approximately ....
, the dinosaur-eating Repenomamus
Repenomamus
Repenomamus is the largest mammal known from the Cretaceous period of Manchuria, and it is the mammal for which there is the best evidence that it fed on dinosaurs. It is not possible to determine if Repenomamus actively hunted live dinosaurs or scavenged dead dinosaurs.-Paleobiology:Repenomamus...
, the flying squirrel
Flying squirrel
Flying squirrels, scientifically known as Pteromyini or Petauristini, are a tribe of 44 species of squirrels .- Description :...
-like Volaticotherium and now the otter-like Castorocauda.