Symmetrodonta is a group of
Mesozoic mammals 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 traditional group of 'symmetrodonts' ranges in age from the latest
Triassic to the Late
Cretaceous, 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 and
zhangheotheriids[ 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 gro ...
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
Pro ...
to the
Campanian,
although
Zhangheotheriidae might be paraphyletic in relation to other forms.
''
Chronoperates'' has been suggested to be a late surviving representative of this clade, offering a ghost lineage extending to the late
Paleocene; however, no recent phylogenetic studies have incorporated it.
Particular sub−groups of Symmetrodonta are better studied, e.g.
Spalacotheriidae
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 hav ...
, which has acute−angled molariform teeth, strongly reduced
talonids, and conspicuous anterior and posterior
cingulids.
Biology
Though some forms like ''
Zhangheotherium'' retain a
Meckelian groove, at least
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 other sof ...
/
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 were ...
diet.
''
Zhangheotherium'' was specialised to a tree-dwelling lifestyle. It shows evidence of tarsal spurs, indicating that, like most non-
therian
Mammaliaformes
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest Extinction, extinct relatives; the group adaptive radiation, radiated from earlier probainognathian cynodonts. It is defined as the clade origin ...
, at least some symmetrodonts were
venomous like the modern
platypus.
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
References
External links
Discovering a Missing Link
{{Taxonbar, from=Q1938798