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Euarchontolgires
The Euarchontoglires families that are discussed here in detail are the Lagomorpha, the Rodentia and the Primates.
Order: Lagomorpha
Placentals arose in the Late Cretaceous starting with small nocturnal shrew-like creatures belonging to the Euarchontoglires. The earliest lagomorphs appeared in the Eocene. Pikas, rabbits and hares were all present in primitive forms. Pikas were especially abundant in the Miocene but declined to reach todays few species.
One of the most known earliest placentals was the shrew-like Zalambdalestes from the Late Cretaceous of Mongolia. It most closely resembled the modern elephant shrew as it had an elongated snout, small body and relatively long legs. Long incisors were placed at the front of the jaw, then a gap before the teeth at the rear of the jaw. The main difference between Zalambdalests and modern shrews is that Zalambdalestes brain was about three quarters of the size. Placental mammals evolved from primitive mammals called marsupials. Marsupials are characterised by a pouch made from skin on their bellies in which they carry their developing young. Placental mammals evolved a placenta and have no need for a pouch. However, the skeletons of primitive placentals such as Zalamdalestes exhibit bones around the hip region which may have supported a pouch. Modern placentals have lost these bones whereas marsupials have kept theirs.
Example Species:
Primitive Lagomorph
Zalambdalestes
Length: 20 cm
Region: Mongolia
Time: Late Cretaceous
Status: Extinct
Rodents are thought to have appeared in the Paleocene (between 65-53 million years ago). One of the three suborders of rodnets, the caviomorphs, first appeared in Africa in the Eocene, From there they spread to South America and grew as large as modern tapirs. The largest species was Telicomys who reached the size of a small rhinoceros ad weighed around a tonne.
Mylagaulids are a group of prehistoric rodents from North America. They were powerful diggers with srtong forelimbs and spade-like paws that terminated in huge claws. Later species had two horns in the centre of their skull, just above the snout. It ws previously thought that the non-horned mylagaulids were females and the horned species were males. However, recent studies have shown that they were in fact different species and actually lack of horns were a primitive state. The mylagaulids were members of the Sciuromorpha Suborder of Rodentia.
Example species:
Primitive rodent
Epigaulus
Length: 30 cm
Region: North America
Time: Late Tertiary
Status: Extinct
Primates are divided into
three groups, the monkeys, the prosimians and the apes that descended
from a common ancestor. The monkeys descended from a group of
mammals called the anthropoidea in the Eocene. The teeth of anthropoids
comprise enlarge canines, flattened molars and premolars. These
dental adaptations allowed the anthropoideans to become adept
at ground level forgaing, whilst other monkeys remained in the
trees. Monkeys such as Theropithecus adapted to grassland
environments and developed a diet of grasses and seeds. Theropithecus
evolved in the late Pliocene and ranged from Africa, through Europe,
to Asia. Theropicthecus oswaldii is the direct ancestor
of the modern gelada of Ethiopia, and was not only considerably
bigger than the gelada but also the largest species of old world
monkey (Catarrhines).
Example species:
Primitive rodent
Theropithecus
Length: 30 cm
Region: North America
Time: Late Tertiary
Status: Extinct
Author: Emma-Louise Nicholls
Last updated: 20th November 2005
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