The lineage of Docodonta evolved prior to the origin of living mammals:
monotremes,
marsupials, and
placentals. In other words, docodontans are outside of the mammalian
crown group, which only includes animals descended from the
last common ancestor of living mammals. Previously, docodontans were sometimes regarded as belonging to
Mammalia, owing to the complexity of their molars and the fact that they possess a
dentary-squamosal jaw joint. However, modern authors usually limit the term "Mammalia" to the crown group, excluding earlier
mammaliaforms like the docodontans. Nevertheless, docodontans are still closely related to crown-Mammalia, to a greater extent than many other early mammaliaform groups such as
Morganucodonta and
Sinoconodon. Some authors also consider docodontans to lie crownward of the order
Haramiyida,
Cladogram based on a phylogenetic analysis of Zhou et al. (2019) focusing on a wide range of mammaliamorphs: A 1956 paper by
Bryan Patterson instead argued that docodontan teeth were impossible to homologize with modern mammals. He drew comparisons to the teeth of
Morganucodon and other "
triconodont" mammaliaforms, which had fairly simple lower molars with a straight row of large cusps. However, re-evaluations of mammaliaform tooth homology in the late 1990s established that docodontans were not closely related to either morganucodonts or therians. Instead, they were found to be similar to certain early "
symmetrodonts", a broad and polyphyletic grouping of mammaliaforms with triangular upper molars. These "symmetrodonts" have three major cusps (c, a, and b) set in a triangular arrangement on their lower molars. These cusps would be homologous to cusps c, a, and g in docodontans, which have a similar size and position. The lingual cusp (cusp X) is prominent in
Woutersia. Unambiguous docodontans are restricted to the
Northern Hemisphere. The oldest docodontan is
Nujalikodon, from the earliest Jurassic (
Hettangian stage) of
Greenland. Many more species appear in the fossil record in the
Middle Jurassic. Very few docodontans survived into the
Cretaceous Period; the youngest known members of the group are
Sibirotherium and
Khorotherium, from the
Early Cretaceous of
Siberia. One disputed docodont,
Gondtherium, has been described from India, which was previously part of the
Southern Hemisphere continent of
Gondwana.
Reigitherium, from the
Late Cretaceous of
Argentina, has previously been described as a docodont, though it is now considered a
meridiolestidan mammal. Some authors have suggested splitting Docodonta into two families (Simpsonodontidae and Tegotheriidae), but the
monophyly of these groups (in their widest form) are not found in any other analyses, and therefore not accepted by all mammal palaeontologists. Cladograms based on phylogenetic analyses focusing on docodontan relationships: Topology of Zhou et al. (2019), based on tooth, cranial, and postcranial traits: •
B. serendipitus Waldman & Savage 1972 •
Castorocauda lutrasimilis Ji et al. 2006 •
D. victor (Marsh 1880) [
Dicrocynodon victor (Marsh 1880);
Diplocynodon victor Marsh 1880] •
D. striatus Marsh 1881 [disputed] •
D. affinis (Marsh 1887) [
Enneodon affinis Marsh 1887] [disputed] •
D. crassus (Marsh 1887) [
Enneodon crassus Marsh 1887;
Ennacodon crassus (Marsh 1887)] [disputed] •
D. superus Simpson 1929 [disputed] •
Docofossor brachydactylus Luo et al. 2015 •
Gondtherium dattai Prasad & Manhas 2007 •
Krusatodon kirtlingtonensis Sigogneau-Russell 2003 •
Microdocodon gracilis Zhou et al. 2019 •
P. major Sigogneau-Russell 2003 [disputed] •
Sibirotherium rossicus Maschenko, Lopatin & Voronkevich 2002 •
Simpsonodon Kermack et al. 1987 •
S. splendens (Kühne 1969) •
S. sibiricus Averianov et al. 2010 •
Tashkumyrodon desideratus Martin & Averianov 2004 •
Tegotherium gubini Tatarinov 1994 ==See also==