Sinoconodon rigneyi is an ancient mammaliamorph or early mammal (depending on systematic approach) that appears in the fossil record of China in the Sinemurian stage of the Early Jurassic period, about 193 million years ago. While in many traits very similar to reptiles, it possessed of a special, secondarily evolved jaw joint between the dentary and the squamosal bones, which had replaced the primitive reptilian one between the articular and quadrate bones, a trait commonly used to define mammals.[1]
Although the animal is closely related to Morganucodon, it is regarded as the most basal of the mammaliaforms.[2] It differed substantially from the more mammalian Morganucodon in its dental and growth habits. Like the reptiles, it was polyphyodont, replacing many of its teeth throughout its lifetime, and it seems to have grown slowly but continuously until its death. Sinoconodon is thus less mammalian than early mammaliaforms like docodonts and morganucodonts.[1] Even the smallest known individuals had already begun the teething cycle of the front teeth, and combined with a poorly ossified jaw, it very probably did not suckle.[3] The combination of reptilian and mammalian features makes it straddle the divide between the two classes anatomically and likely ecologically. There are simply no animals like it alive today.
Taxonomy[]
Sinoconodon was named by Patterson and Olson in 1961. Its type is Sinoconodon rigneyi. It was assigned to Triconodontidae by Patterson and Olson in 1961; to Triconodonta by Jenkins and Crompton in 1979; to Sinoconodontidae by Carroll in 1988; to Mammaliamorpha by Wible in 1991; to Mammalia by Luo and Wu in 1994; to Mammalia by Kielan-Jaworowska et al. in 2004; and to Mammaliaformes by Luo et al. in 2001 and Bi et al. in 2014.
Phylogeny[]
| Mammaliaformes |
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Sinoconodon[11][12] differs from all nonmammalian cynodonts in the presence of a promontorium, an enlarged anterior lamina, and the floor of the trigeminal ganglion. Sinoconodon shares several derived characters with other mammals. The most distinguished are the expansion of the brain vault in the parietal region, complete ossification of the medial wall of the orbit, a large dentary condyle, and a concave glenoid fossa in the squamosal. These characters suggest that Sinoconodon and other mammals form a monophyletic group. In addition, Sinoconodon developed some autapomorphic characters: a large occipital condyle; and, relative to skull length, postdentary bones more reduced in size than in the other known Liassic mammals. These characters suggest that Sinoconodon is the sistergroup to a taxon that includes all other mammals because Sinoconodon lacks a number of diagnostic apomorphies shared by Morganucodon, Dinnetherium, and other mammals.
Relationships[]
The evolutionary transition to the mammalian type of jaw joint was associated with changes in body size, structure of the ear, structure of the skull, structure of the teeth, and, consequently, the type of diet that distinguishes mammals from other vertebrates. Morganucodon can still be regarded as the best known of the earliest mammals. The skull, lower jaw, and postcrania of this small mouse-sized mammal are well preserved and have provided the basis for many ideas of mammal origins.
Sinoconodon presents morphological data that forces a modification of ideas of mammal origins based primarily on Morganucodon. Sinoconodon is known only from the skull and lower jaws. It is younger geologically than some Morganucodon, but Sinoconodon has many features which are more primitive than Morganucodon. Particularly significant is the dentition of Sinoconodon, in which the postcanine tooth row consists of five multicuspid trenchant teeth with only the vestiges of cingula. These teeth do not precisely occlude with one another. They do not look like typical mammalian teeth, which do occlude precisely and have distinct cingula and cusps offset from a single longitudinal row. Indeed precise occlusion is thought to have evolved very early in the evolution of mammals, in Morganucodon itself.