Symmetrodonta is a group of
Mesozoic
The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
mammals
A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...
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
The Triassic ( ; sometimes symbolized đ) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
to the Late
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
, 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 mammalsâearly 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 (âacute-angled symmetrodontsâ) that seem to form a true
monophyletic
In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade â that is, a grouping of organisms which meets these criteria:
# the grouping contains its own most recent co ...
group, and lasted from the
Early Cretaceous
The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
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. Campa ...
,
although
Zhangheotheriidae
Zhangheotheriidae is a possibly paraphyletic family of "symmetrodont" mammals that is currently known from Early Cretaceous deposits in China and Russia. Six genera are currently recognized, ''Anebodon'', ''Kiyatherium'', ''Maotherium'', ''Ningch ...
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
The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
; however, no recent phylogenetic studies have incorporated it.
Particular subâgroups 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 ha ...
, which has acuteâangled 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 nutrition and energy requirements are met by consumption of animal tissues (mainly mu ...
/
insectivorous
A robber fly eating a hoverfly
An insectivore is a carnivorous animal or plant which 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'' was specialised to a tree-dwelling lifestyle. It shows evidence of tarsal spurs, indicating that, like most non-
theria
Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...
n
Mammaliaformes
Mammaliaformes ("mammalian forms") is a clade of synapsid tetrapods that includes the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts during the Late Triassic. It is defined as ...
, 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 sti ...
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 monotypi ...
.
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 (geology), Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synaps ...
References
External links
Discovering a Missing Link
{{Taxonbar, from=Q1938798