Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
mammals
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), f ...
and mammal-like synapsids characterized by the triangular aspect of the molars when viewed from above, and the absence of a well-developed
talonid
The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...
. The traditional group of 'symmetrodonts' ranges in age from the latest
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest perio ...
to the Late
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
, but most research in the last 20-30 years has concluded that they are not a true taxonomic group, but include several unrelated branches of the mammal tree. Despite this, the name is still used informally by some researchers for convenience, usually restricted to the
spalacotheriids
Spalacotheriidae is a family of extinct mammals belonging to the paraphyletic group 'Symmetrodonta'. They lasted from the Early Cretaceous to the Campanian in North America, Europe, Asia and North Africa.
Spalacotheriids are characterised by h ...
and
zhangheotheriids
Zhangheotheriidae is a possibly paraphyletic family of "symmetrodont" mammals that is currently known from Early Cretaceous deposits in China and Russia. Five genera are currently recognized, '' Anebodon'', '' Kiyatherium'', '' Maotherium'', '' ...
Martin, T., 2018. 6. Mesozoic mammals—early mammalian diversity and ecomorphological adaptations. In Mammalian evolution, diversity and systematics (pp. 199-300). De Gruyter..
There are some symmetrodonts with acutely-triangulated molar cusps (“acute-angled symmetrodonts”) that seem to form a true
monophyletic
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...
group, and lasted from the
Early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous ( chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pr ...
to the
Campanian
The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campani ...
, although
Zhangheotheriidae
Zhangheotheriidae is a possibly paraphyletic family of "symmetrodont" mammals that is currently known from Early Cretaceous deposits in China and Russia. Five genera are currently recognized, '' Anebodon'', '' Kiyatherium'', '' Maotherium'', '' ...
might be paraphyletic in relation to other forms. ''
Chronoperates
''Chronoperates'' (meaning "time wanderer" in Greek) is an extinct genus of mammal whose remains have been found in a late Paleocene deposit in Alberta, Canada. It is represented by the type species ''Chronoperates paradoxus'' and known only ...
'' has been suggested to be a late surviving representative of this clade, offering a ghost lineage extending to the late
Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''palai ...
; however, no recent phylogenetic studies have incorporated it.
Particular sub−groups of Symmetrodonta are better studied, e.g. Spalacotheriidae, which has acute−angled molariform teeth, strongly reduced
talonid
The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...
s, and conspicuous anterior and posterior
cingulid
A cingulid is a term used when describing teeth, it refers to a ridge that runs around the base of the crown of a lower tooth (the equivalent on the upper teeth is the cingulum). The presence or absence of a cingulid is often a diagnostic featur ...
s.
Biology
Though some forms like ''
Zhangheotherium
''Zhangheotherium'' is a genus of symmetrodont, an extinct order of mammals. Previously known from only the tall pointed crowned teeth, ''Zhangheotherium'', described from Liaoning Province, China, fossils in 1997, is the first symmetrodont k ...
'' retain a
Meckelian groove The Meckelian groove (or Meckel's groove, Meckelian fossa, or Meckelian foramen, or Meckelian canal) is an opening in the medial (inner) surface of the mandible (lower jaw) which exposes the Meckelian cartilage.Spalacotheriinae lost it, acquiring modern ear anatomy. Their deciduous canines and premolars as well as long lower jaw indicate a
carnivorous
A carnivore , or meat-eater ( Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and oth ...
/
insectivorous
A robber fly eating a hoverfly
An insectivore is a carnivorous animal or plant that eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects.
The first vertebrate insectivores we ...
diet.
''
Zhangheotherium
''Zhangheotherium'' is a genus of symmetrodont, an extinct order of mammals. Previously known from only the tall pointed crowned teeth, ''Zhangheotherium'', described from Liaoning Province, China, fossils in 1997, is the first symmetrodont k ...
'' was specialised to a tree-dwelling lifestyle. It shows evidence of tarsal spurs, indicating that, like most non-
theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...
n
Mammaliaformes
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent ...
, at least some symmetrodonts were
venomous
Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a s ...
like the modern
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 monoty ...
.
One species, '' Spalacotheridium noblei'', is notable for its small size. It is one of the smallest known mammals. Each individual molar is little more than 0.25 mm across.
See also
*
Evolution of mammals
The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked l ...