Sinoconodon
Encyclopedia
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. Similar to reptile
s, it replaced many of its teeth (its incisor
s, canines
, and postcanines) throughout its lifetime (polyphydont
), and it seems to have grown slowly but continuously until its death.
Therefore, Sinoconodon is generally considered more primitive (less mammalian) than related Mammaliaformes, including docodonts
and morganucodonts
, which appear a few million years later. Some label it the most primitive mammal taxon
, while others do not label it a mammal at all.
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...
that appears in the fossil record in the late Triassic
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...
period, about 208 million years ago. Although the animal seems more related to 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...
than anything else, it differed substantially from other 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...
in its dental and growth habits. Similar to reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, it replaced many of its teeth (its incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s, canines
Canine tooth
In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, fangs, or eye teeth, are relatively long, pointed teeth...
, and postcanines) throughout its lifetime (polyphydont
Polyphydont
A polyphyodont is an animal whose teeth are continuously replaced. Most toothed fishes are polyphyodonts. Diphyodonts, on the other hand, are characterized by having only two successive sets of teeth. Reptiles and most other vertebrates are also polyphyodonts, mammals being the main exception....
), and it seems to have grown slowly but continuously until its death.
Therefore, Sinoconodon is generally considered more primitive (less mammalian) than related Mammaliaformes, including docodonts
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...
and morganucodonts
Morganucodonta
The morganucodonts are an extinct group of important animals that seem to represent the earliest of mammals. These were shrew-sized, likely warm-blooded and possibly possessed mammary glands....
, which appear a few million years later. Some label it the most primitive mammal taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
, while others do not label it a mammal at all.