Basal Mammals

Adelobasileus

Adelobasileus from the Late Triassic of Texas, at 225 Ma, represents the earliest mammalian candidate. Ironically, Adelobasileus is not defined as a mammal by its jaw or teeth. Known only from a partial skull, it possesses a number of mammalian braincase features (Benton, 2005, pp. 298-300).

Haramiyidans

Haramiyidans (order Haramiyida) represent the next earliest known mammals and range from the Late Triassic to the Mid Jurassic. Haramiyidans first appear in the Late Triassic of Germany (Kemp, 2005, p. 140). Haramiyavia represents one of the most complete haramiyidan fossils from the Late Triassic of Greenland. Haramiyavia possesses a postcranial skeleton similar to morganucodonts; however, the teeth are similar to mulituberculates (Rose, 2006, p. 51). The similarity in dentition has led many to argue that haramiyidans are basal relatives to the multituberculates. Haramiyidans teeth indicate they were small herbivores or at least omnivores (Kemp, 2005, p. 140). At present, Haramiyidans are the earliest known herbivorous mammals.

Morganucodonts

Morganucodonts (order Morganucodonta) is a well-known group of early Mesozoic mammals. Morganucodonts were shrew-sized nocturnal insectivores. Morganucodon is a weasel like morganucodon of the Late Triassic and Early Jurassic of England and China. Morganucodon possesses a mammalian dentary/squamosal jaw joint; however, it also possesses a reduced, but functional, quadrate/articular jaw joint (Rose, 2006, p. 52). Thus, Morganucodon had a double jaw articulation. This implies that the malleus and incus were not yet a part of the middle ear. The differentiated teeth of Morganucodon are diphyodont, meaning they have two sets of teeth during life. Remember this is in contrast with early amniotes and reptiles that have undifferentiated teeth that are constantly replaced throughout life (polyphydont). However, its teeth did not show precise occlusion. Morganucodon is known only from skulls, but the skeleton of its close relative Megazostrodon of South Africa is known. The skeleton of Megazostrodon exhibits a mosaic of mammal and primitive synapsid characteristics. Megazostrodon had its ribcage restricted to the thorasic vertebrae. Megazostrodon had a pelvic girdle that was more mammal-like; however, their pectoral girdle was more like early synapsids.

Sinoconodon

Sinoconodon from the Late Triassic of China, at (208 Ma), may also occupy a basal position in mammalian phylogeny. Sinoconodon is an intermediate form in that it possessed both mammal-like and early amniote characteristics. Sinoconodon has braincase features like Adelobasileus and possesses a fully developed dentary/squamosal jaw joint. Sinoconodon also possesses some non-mammalian features, which include serial tooth replacement (polyphydont) and sustained growth throughout life (Kemp, 2005, p. 142).

Keuhneotherium

Kuehneotherium from the Lower Jurassic of South Wales is known mostly from isolated teeth. Kuehneotheriids possess teeth with a triangular cusp pattern that may represent a precursor to the tribosphenic molars that help to define modern mammals. Kuehneotherium also retained some primitive traits such as a double jaw articulation (Rose, 2006, p. 54).

Hadrocodium

Hadrocodium is known from a single shrew-sized skull from the Lower Jurassic of China (195Ma). Hadrocodium is one of the smallest known mammals and is estimated to have weighed only 2 g (Rose, 2006, p. 55). Hadrocodium possesses primitive teeth similar to morganucodonts. However, it has a single jaw articulation between the dentary and squamosal bones, which suggests that the quadrate and articular bones had become ear ossicles (Kemp, 2005, p. 149).

Docodonts

Docodonts (order Docodonta) are known only from Jurassic deposits. Some of the first docodonts were found in the Morrison formation of Wyoming and Colorado along with the bones of large sauropod dinosaurs (Rose, 2006, p. 55). Docodonts have broad, rectangular cheek teeth that show precise occlusion that may indicate an omnivorous diet (Kemp, 2005, p. 147). Haldanodon is known from a complete skeleton found in the Late Jurassic Guimarota lignites of Portugal. The skeletal features of this organism suggest that it may have been adapted for burrowing. The fact that it is found in lignites (swamp deposits), indicate a semi-aquatic lifestyle. Castorocauda, from the Mid Jurassic of China, is the largest docodont at half a meter in length. Castorocauda had a skeleton adapted for burrowing and swimming. Castorocauda is also important because it is the first mammal fossil preserving evidence of fur (Rose, 2006, p. 56).

Multituberculates

Multituberculates (order Multituberculata) are an extinct group of rodent-like organisms that have the longest evolutionary history of any mammalian lineage. Multituberculates get their name from their large grinding molars that have rows of cusps or tubercles. Multituberculates first appear in the Mid Jurassic and evolved into many forms, which ranged from mice to beaver sized organisms. Many of these organisms had blade-like teeth that may have been used to eat hard seeds. Multituberculate hip structure suggests that they gave birth to undeveloped young like marsupials. Multituberculates had a single dentary/squamosal jaw joint and true inner ear ossicles. Kermackodon and Hahnotherium from the Mid Jurassic of England are the oldest known multituberculates (Rose, 2006, p. 56). Ptilodus was a squirrel-like organism that lived in trees. It had a prehensile tail and an ankle structure designed to allow moving down trees headfirst. Lambdopsalis was a beaver-sized organism that lived in burrows like prairie dogs do today. Remarkable carnivore coprolites from China contain the bones and fur impressions of Lambdopsalis (Kemp, 2005, p. 160). Multituberculates possess many rodent-like features, which represent convergent evolution as they predate the first rodents by 180 million years. Multituberculates were abundant in many Mesozoic and Cenozoic communities in the northern continents, but went extinct in the Oligocene, perhaps even displaced by true rodents that had evolved and diversified by this time.

Triconodonts

The order Eutriconodonta is a taxon that represents a diverse group of extinct mammals that span from the Mid Jurassic to Late Cretaceous. Triconodonts were rat to cat-sized mammals that lie at the core of this group. Triconodonts had the dentary/squamosal jaw joint and the three inner ear ossicles. Eutriconodonts are named for their teeth, which have three linear cusps on their molars. The lower molars were interlocked by a unique tongue-in-groove articulation. Eutriconodonts had the derived mammalian pectoral girdle (limbs tucked underneath the body), but retained the ancestoral pelvic girdle (sprawling hind limbs). Jeholodens is a Cretaceous-aged triconodont mammal known from the Liaoning Province of China. A single complete skeleton represents Jeholodens. Skeletal evidence indicates that this small primitive mammal, like many Mesozoic mammals, was a nocturnal insectivore. Repenomamus is the largest mammal known from the Cretaceous of China. Repenomamus was a carnivore up to 1 meter long. One specimen of Repenomamus had the partial skeletal remains of a juvinelle Psittocosaurus preserved within the stomach region (Rose, 2006, p. 62). Psittocosaurus was a ceratopsian dinosaur. This incredible fossil is the first evidence of a mammal preying on a dinosaur.

Symmetrodonts & Dryolestids

Several closely related groups of Mesozoic mammals exhibit molar teeth with a triangular cusp pattern. The symmetrodonts and eupantotheres (Dryolestoidea and Peramura) represent mammals that are closely related to the therians (marsupials and placentals). We will briefly discuss two of these groups, the dryolestids (Order Dryoletida) and symmetrodonts (Order Symmetrodonta).

Symmetrodonts were shrew to mouse sized and are known from the Early Jurassic to Late Cretaceous. Symmetrodonts are believed to be at the base of the therian radiation because of the triangular cusp pattern on their molars. Zhangheotherium is one of the few symmetrodonts known from almost a complete skeleton. Zhangheotherium lived in China during the Early Cretaceous and possessed skeletal characteristics intermediate between monotremes and therians.

Dryolestids, the most diverse eupantotheres, range from the Late Jurassic to the Late Cretaceous. Dryolestids have a more advanced triangular cusp pattern on their molar teeth than the symmetrodonts and possessed three inner ear bones. It is believed by many that the ancestors to modern therians can be found among the dryolestids.

Tribosphenic Molar & Modern Mammals

Mammal groups examined thus far tend to have cheek teeth with cusps oriented either in a linear fashion or a primitive triangular fashion. When linear, the cusps on upper molars fit between the cusps on lower molars. When triangular, the cusps on upper molars fit into V-shaped valleys between the tricuspid patterns on the lower molars. The evolution of the tricuspid pattern is important because it represents an innovation in processing food.

Sometime in the early Cretaceous the advanced triangular cusp pattern that defines modern mammals, the tribosphenic tooth, appears. The linear and primitive triangular tricuspid patterns represent cheek teeth that are good for cutting and tearing, but not crushing. The more advanced tribosphenic tooth has a triangular cusp pattern that creates occlusion surfaces good for crushing or grinding, like a pestle and mortar. The tribosphenic tooth is defined by the presence of a large cusp on the upper molars called the protocone. The protocone of the upper molar works against a basined area named the talonid on the corresponding lower molar. The protocone thus acts as a pestle, while the talonid acts as the mortar. The tribosphenic molar provided a basic form that would later be modified into the wide variety of dentitions exhibited by therian mammals (marsupials and placentals). This dental structure allowed mammals to expand into a wide variety of specialized dietary niches (Rose, 2006, p. 67).

During the entire reign of the dinosaurs mammals remained small never getting much bigger than a cat. The evolutionary biologist Stephen J. Gould (1988) commented, “Mammals were relatively rare…They lived under the foot of dinosaurs, in the nooks and crannies of a dinosaur world.” However, as we have seen, a succession of characteristics that define modern mammals evolved among the Mesozoic mammals. Fossil representatives of monotremes, marsupials and placental mammals are known from the Cretaceous. The extinction event at the end of the Cretaceous opened up a multitude of niches. Mammals had evolved many characteristics during the Mesozoic that would allow their descendents to evolve into a diversity of forms that could occupy many of these empty niches.

Benton, M.J. (2005). Vertebrate Palaeontology [3rd edition]. Main: Blackwell Publishing.

Gould, S.J. (1988). The Infinite Voyage: The Great Dinosaur Hunt. Stamford, CT: Vestron Video.

Kemp, T.S. (2005). The Origin and Evolution of Mammals. New York: Oxford University Press.

Rose, K.D. (2006). The Beginning of the Age of Mammals. Baltimore: The Johns Hopkins University Press.