Dryolestoidea

Dryolestoidea Temporal range: Middle Jurassic–Early Miocene
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Legion:	Cladotheria
Superorder:	†Dryolestoidea Butler 1939
Orders
Dryolestida Amphitheriida

Dryolestoidea is an extinct clade of Mesozoic mammals that only contains two orders. It has been suggested that this group contained the ancestors of modern therian mammals.

Characteristics

Dryolestids are mostly represented by teeth, fragmented dentaries and parts of the rostrum. The Jurassic forms retained a coronoid and splenial, but the Cretaceous forms lack these. Another primitive feature is the presence of a Meckelian groove (the Cretaceous forms have a reduced version).^[1]

Tooth enamel evolved differently in marsupials and eutherians. In a first phase, during the late Triassic and Jurassic, prisms separated from the interprismatic matrix, probably independently in several Mesozoic mammal lineages. More derived enamel types evolved in a second phase, during the Tertiary and Quaternary, but without replacing the old prismatic enamel, instead forming various combinations of three-dimensional structures (called schmelzmuster). Dryolestid dentition is thought to resemble the primitive mammalian dentition before the marsupial-eutherian differentiation and dryolestids are candidates to be the last common ancestor of the two mammalian subclasses.^[2]

Distribution

Dryolestoids are known from the Jurassic through Early Cretaceous of the northern hemisphere (North America, Eurasia, and North Africa) and from the Late Cretaceous through Paleocene of South America.^[1] Drylestoids are very rarely found in the Cenozoic, as are the few other Mesozoic mammals with later descendants, such as multituberculates, monotremes, and gondwanatheres.^[3]

The oldest undisputed fossils of Dryolestidae were found in the Guimarota coal mine near Leiria, Portugal, the largest known deposit of Jurassic mammals. The tooth pattern of these fossils — 8–9 mesiodistally compressed molars with small, cusp-like talonids — distinguish them from tribosphenic mammals, and they form a sister group of the latter and Peramus together with Amphitherium, Paurodontidae, and Henkelotheriidae.^[4]

The youngest fossil in the northern hemisphere is Crusafontia cuencana from early Cretaceous of Uña and Galve, Spain, though a fragmentary lower molar from the late Cretaceous Mesaverde Formation in Wyoming has been attributed to Dryolestidae. In South America, dryolestids survived until the late Cretaceous when they underwent a remarkable radiation.^[4]

Notes

References