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 (Meridiolestidans lost it altogether). A fundamentally modern ear is known in at least Dryolestes and mesungulatids.
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.
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 to the Miocene of South America. Drylestoids are very rarely found in the Cenozoic, as are the few other Mesozoic mammals with later descendants, such as multituberculates, monotremes, and gondwanatheres.
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.
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, by contrast, dryolestoids thrived in the Late Cretaceous, diversifying in a myriad of forms such as the saber-toothed Cronopio or the herbivorous mesungulatids, becoming some of the most ecologically diverse Mesozoic South American mammals. At least two major Late Cretaceous South American dryolestoid clades are known: Meridiolestida and a clade similar to Dryolestes itself, Groebertherium.
With the advent of the Cenozoic, dryolestoids declined drastically in diversity, with only Peligrotherium being known from the Palaeocene. The exact reasons for this decline aren't clear; most likely they simply did not recover from the K-Pg event. Nonetheless, dryolestoids would continue to survive until the Miocene, from when Necrolestes is known; a gap of 50 million years exists between it and Peligrotherium.
Amphitheriida is an order of Mesozoic mammals restricted to the Middle Jurassic of Britain. They were closely related to the Dryolestids but possessed five molars instead of the usual four in Dryolestida, (with the exception of the family Dryolestidae whose members possessed between 8 and 9 molars). The Amphitheriida contains one family, the Amphitheriidae.Austrotriconodon
Austrotriconodon is a mammal genus from the Campanian and Maastrichtian of South America. It currently contains only the type species, A. mckennai. Originally assumed to be a eutriconodont, more recent studies have recovered it as a meridiolestidan dryolestoid.Brandoniidae
Brandoniidae is a family of meridiolestidan dryolestoids indigenous to the Late Cretaceous of South America. Most are known from the Los Alamitos Formation, but also occur in other argentinian and brazilian sites.Cladotheria
Cladotheria is a group (legion) of mammals that includes the ancestor of Dryolestoidea, Peramuridae and Zatheria (living therians plus all of its ancestors).Coloniatherium
Coloniatherium is a dryolestoid mammal from the Late Cretaceous of Argentina. The single species, Coloniatherium cilinskii, was a large member of the family Mesungulatidae.Comodon
Comodon is an extinct genus of Late Jurassic mammal from the Morrison Formation.
Present in stratigraphic zone 5.Cronopio (mammal)
Cronopio is an extinct genus of dryolestoid mammals known from the Río Negro region of Argentina.Dryolestida
Dryolestida is an extinct order of mammals; most of the members are mostly known from the Jurassic to Paleogene, with one member, Necrolestes, surviving as late as the early Miocene. It has been suggested that these mammals are either the possible ancestors of therian mammals or an offshoot from the same evolutionary line. It is also believed that they developed a fully mammalian jaw and also had the three middle ear bones. Other than that, not much is known about them, this is because their fossils are made up mostly of jaw and tooth remains.
Dryolestids were formerly considered part of Pantotheria and/or Eupantotheria. The clade Quirogatheria, erected by José Bonaparte in 1992, is often used as a synonym for Dryolestida. Originally, Quirogatheria was meant to include Brandoniidae, but this family is now included with the dryolestids.
It has been suggested on the basis of morphological evidence that they may still be extant in the form of marsupial moles. However, a molecular study places marsupial moles within Euaustralidelphia, though several problems with this assessment have been addressed.Groebertherium
Groebertherium is a genus of dryolestoid mammal from the Late Cretaceous Los Alamitos and Allen Formations of Argentina. It is not closely related to other contemporary dryolestoids, all of which are part of the clade Meridiolestida.Holotheria
Holotheria are a diverse group of mammals that are descendants of the last common ancestor of Kuehneotherium and Theria (the group that includes marsupials and placental mammals).Leonardus
Leonardus is an extinct mammal genus from the Late Cretaceous (Late Santonian to Maastrichtian) of South America. It is a meridiolestidan dryolestoid, closely related to the also Late Cretaceous Cronopio and the Miocene Necrolestes, and potentially also modern marsupial mole.Lourinhã Formation
The Lourinhã Formation is a fossil rich geological formation in western Portugal, named for the municipality of Lourinhã. The formation is Late Jurassic in age (Kimmeridgian/Tithonian) and is notable for containing a fauna similar to that of the Morrison Formation in the United States and the Tendaguru beds in Tanzania. The stratigraphy of the formation and the basin in general is disputed, with the constituent member beds belonging to the formation varying between different authorsBesides the fossil bones, Lourinhã Formation is well known for the fossil tracks and fossilized dinosaur eggs.The Lourinhã Formation includes several lithostratigraphic units, such as Praia da Amoreira-Porto Novo Members and the Sobral Unit.Meridiolestida
Meridiolestida is a clade of non-therian mammals from South America and Africa, and potentially also Australia. It is generally classified within Dryolestida, barring one study recovering them as the sister taxa to spalacotheriid symmetrodonts. However, more recent studies have stuck to the dryolestoid interpretation. They differ from northern dryolestoids in the absence of a parastylar hook on the molariform teeth and the lack of a Meckelian groove.Necrolestes
Necrolestes ("grave robber" or "thief of the dead") is an extinct genus of non-therian mammals, which lived during the Early Miocene in what is now Argentine Patagonia. It contains two species, N. patagonensis and N. mirabilis, and is the most recent known genus of dryolestoid. The type species N. patagonensis was named by Florentino Ameghino in 1891 based on remains found by his brother, Carlos Ameghino in Patagonia. Fossils of Necrolestes have been found in the Sarmiento and Santa Cruz Formations.Palaeosinopa
Palaeosinopa is an extinct genus of semi-aquatic, non-placental eutherian mammals belonging to the family Pantolestidae. Their diet consisted of other semi-aquatic life forms.Plethorodon
Plethorodon is an extinct genus of tillodont that lived during Early to Late Paleocene. The type species is P. qianshanensis. which known from partial skull and upper teeth that had been discovered by Huang and Zheng at 1987 at Qianshan, Anhui Province, China.Theriiformes
Theriiformes is a subclass of mammals. The term was coined in 1997 by McKenna & Bell in their classification of mammals. In the strict sense, it is defined as all mammals more closely related to therians than to monotremes.Tribosphenida
Tribosphenida is a group (infralegion) of mammals that includes the ancestor of Hypomylos, Aegialodontia and Theria (the last common ancestor of marsupials and placentals plus all of its descendants). Its current definition is more or less synonymous with Boreosphenida.Zatheria
Zatheria is a group (sublegion) of mammals that includes the common ancestor of Arguimuridae, Vincelestidae, Peramuridae and Tribosphenida (living therians plus all of its ancestors).