Dryolestoidea

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Dryolestoidea
Temporal range: Middle JurassicEarly Miocene
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Legion: Cladotheria
Superorder: Dryolestoidea
Butler 1939
Orders

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[edit]

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[edit]

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[edit]

  1. ^ a b Kielan-Jaworowska, Cifelli & Luo 2004, pp. 14, 375, 379–380
  2. ^ von Koenigswald 2000, p. 107
  3. ^ Rose 2006, pp. 335–6
  4. ^ a b Martin 1997, Introduction

References[edit]

  • Butler, PM (December 1939). "The Teeth of the Jurassic Mammals". Proceedings of the Zoological Society of London. B 109 (3-4): 329–356. doi:10.1111/j.1096-3642.1939.tb00719.x. 
  • Kielan-Jaworowska, Zofia; Cifelli, Richard L; Luo, Zhe-Xi (2004). Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure. New York: Columbia University Press. ISBN 978-0-231-11918-4. 
  • Martin, Thomas (1997). "Tooth Replacement in Late Jurassic Dryolestidae (Eupantotheria, Mammalia)". Journal of Mammalian Evolution 4 (1): 1–18. doi:10.1023/A:1027300726126. OCLC 360227463. 
  • Rose, Kenneth David (2006). The beginning of the age of mammals. Baltimore: JHU Press. ISBN 0801884721. 
  • von Koenigswald, Wighart (2000). "Two different strategies in enamel differentiation: Marsupialia versus Eutheria". In Teaford, Mark F; Smith, Moya Meredith; Ferguson, Mark WJ. Development, Function and Evolution of Teeth. Cambridge University Press. ISBN 978-0-511-06568-2.