Indohyus
Encyclopedia
Indohyus is a genus
of extinct artiodactyl known from Eocene
fossil
s in Asia, purported to be approximately 48 million years old. A December 2007 article in Nature
by Thewissen et al. used an exceptionally complete skeleton of Indohyus from Kashmir
to indicate that raoellids
may be the "missing link
" sister group to whales (Cetacea
). All other Artiodactyla are "cousins" of these two groups. δ18O
values and osteosclerotic bones indicate that the raccoon
-like or chevrotain
-like Indohyus was habitually aquatic, but δ13C
values suggest that it rarely fed in the water. The authors suggest this documents an intermediate step in the transition back to water completed by the whales, and suggests a new understanding of the evolution of cetaceans
.
The fossils were discovered among rocks that had been collected more than 30 years ago in Kashmir
by the Indian geologist A Ranga Rao who found a few teeth and parts of a jawbone, but when he died many rocks had yet to be broken open. Ranga Rao's widow gave the rocks to Professor Thewissen, who was working on them when his technician accidentally broke one of the skulls they had found and Thewissen recognised the ear structure of the auditory bulla
, formed from the ectotympanic bone in a shape which is highly unusual and only resembles the skulls of whales and the earlier land creature Pakicetus
.
About the size of a raccoon or domestic cat, this herbivorous deer-like creature shared some of the traits of whales, and showed signs of adaptations to aquatic life, including a thick and heavy outer coating to bones which is similar to the bones of modern creatures such the hippopotamus
, and reduces buoyancy so that they can stay underwater. This suggests a similar survival strategy to the African mousedeer
or water chevrotain
which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.
However, not all paleontologists are firmly persuaded that Indohyus is the transitional fossil
that cetacean-origin experts were looking for. ScienceNOW, a daily news feature of the journal Science
, notes that a team set to publish in the journal Cladistics
postulates an extinct group of carnivorous mammals called "mesonychid
s" as more closely related to cetacea
ns. Additionally, the ScienceNOW article notes that "cetaceans are so different from any other creature that researchers haven’t been able to agree which fossil relatives best represent their nearest ancestors."
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 extinct artiodactyl known from Eocene
Eocene
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...
fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s in Asia, purported to be approximately 48 million years old. A December 2007 article in Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
by Thewissen et al. used an exceptionally complete skeleton of Indohyus from Kashmir
Kashmir
Kashmir is the northwestern region of the Indian subcontinent. Until the mid-19th century, the term Kashmir geographically denoted only the valley between the Great Himalayas and the Pir Panjal mountain range...
to indicate that raoellids
Raoellidae
Previously grouped with Helohyidae, Raoellidae is now a family in the Suborder Cetancodonta. It is found in Eocene of South and Southeast Asia....
may be the "missing link
Missing Link
Missing link is a nonscientific term for any transitional fossil, especially one connected with human evolution; see Transitional fossil - Missing links and List of transitonal fossils - Human evolution.Missing Link may refer to:...
" sister group to whales (Cetacea
Cetacea
The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...
). All other Artiodactyla are "cousins" of these two groups. δ18O
Oxygen-18
Oxygen-18 is a natural, stable isotope of oxygen and one of the environmental isotopes.18O is an important precursor for the production of fluorodeoxyglucose used in positron emission tomography...
values and osteosclerotic bones indicate that the raccoon
Raccoon
Procyon is a genus of nocturnal mammals, comprising three species commonly known as raccoons, in the family Procyonidae. The most familiar species, the common raccoon , is often known simply as "the" raccoon, as the two other raccoon species in the genus are native only to the tropics and are...
-like or chevrotain
Chevrotain
Chevrotains, also known as mouse deer, are small ungulates that make up the family Tragulidae, the only members of the infraorder Tragulina. There are 10 living species in three genera, but there are also several species only known from fossils...
-like Indohyus was habitually aquatic, but δ13C
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...
values suggest that it rarely fed in the water. The authors suggest this documents an intermediate step in the transition back to water completed by the whales, and suggests a new understanding of the evolution of cetaceans
Evolution of cetaceans
The cetaceans are marine mammal descendants of land mammals. Their terrestrial origins are indicated by:* Their need to breathe air from the surface;* The bones of their fins, which resemble the limbs of land mammals...
.
Kashmir
Kashmir is the northwestern region of the Indian subcontinent. Until the mid-19th century, the term Kashmir geographically denoted only the valley between the Great Himalayas and the Pir Panjal mountain range...
by the Indian geologist A Ranga Rao who found a few teeth and parts of a jawbone, but when he died many rocks had yet to be broken open. Ranga Rao's widow gave the rocks to Professor Thewissen, who was working on them when his technician accidentally broke one of the skulls they had found and Thewissen recognised the ear structure of the auditory bulla
Auditory bulla
The auditory bulla is a hollow bony structure on the ventral, posterior portion of the skull of placental mammals that encloses parts of the middle and inner ear. In most species, it is formed by the tympanic part of the temporal bone.In extant primates, the structure is found in tarsiers,...
, formed from the ectotympanic bone in a shape which is highly unusual and only resembles the skulls of whales and the earlier land creature Pakicetus
Pakicetus
Pakicetus is a genus of extinct terrestrial carnivorous mammal of the family Pakicetidae which was endemic to Pakistan from the Eocene .Pakicetus existed for approximately...
.
About the size of a raccoon or domestic cat, this herbivorous deer-like creature shared some of the traits of whales, and showed signs of adaptations to aquatic life, including a thick and heavy outer coating to bones which is similar to the bones of modern creatures such the hippopotamus
Hippopotamus
The hippopotamus , or hippo, from the ancient Greek for "river horse" , is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae After the elephant and rhinoceros, the hippopotamus is the third largest land mammal and the heaviest...
, and reduces buoyancy so that they can stay underwater. This suggests a similar survival strategy to the African mousedeer
Chevrotain
Chevrotains, also known as mouse deer, are small ungulates that make up the family Tragulidae, the only members of the infraorder Tragulina. There are 10 living species in three genera, but there are also several species only known from fossils...
or water chevrotain
Hyemoschus aquaticus
The water chevrotain , also known as the fanged deer, is a small ruminant found in tropical Africa. It is the largest of the ten species of chevrotains, evolutionarily primitive even-toed ungulates which are similar to deer but are barely larger than small dogs...
which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.
However, not all paleontologists are firmly persuaded that Indohyus is the transitional fossil
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
that cetacean-origin experts were looking for. ScienceNOW, a daily news feature of the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
, notes that a team set to publish in the journal Cladistics
Cladistics (journal)
Cladistics is a bimonthly peer-reviewed scientific journal publishing research in cladistics. It is published by Wiley-Blackwell on behalf of the Willi Hennig Society. Cladistics publishes papers relevant to evolution, systematics, and integrative biology...
postulates an extinct group of carnivorous mammals called "mesonychid
Mesonychid
Mesonychia are an extinct order of medium to large-sized carnivorous mammals that were closely related to artiodactyls and to cetaceans...
s" as more closely related to cetacea
Cetacea
The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...
ns. Additionally, the ScienceNOW article notes that "cetaceans are so different from any other creature that researchers haven’t been able to agree which fossil relatives best represent their nearest ancestors."
Taxonomy
Two species have been described.- Genus Indohyus
- Indohyus indirae
- Indohyus major