Mammaliaformes

Ecological niches in the Mesozoic

There is still some truth in the "small, nocturnal insectivores" stereotype, but recent finds show that proto-mammals gradually developed a variety of lifestyles. For example:

• Castorocauda, a member of Docodonta lived in the Middle Jurassic about 164 million years ago (mya). It was about 42.5 cm (16.7 in) long, weighed 500–800 g (18–28 oz), had limbs that were adapted for swimming and digging and teeth adapted for eating fish. Another docodont, Haldanodon, also had semi-aquatic habits. Aquatic tendencies were probably common among docodonts, since they lived in wetland environments.
• Multituberculates are allotherians that survived for over 125 mya (from mid-Jurassic, about 160 mya, to late Eocene, about 35 mya) are often called the "rodents of the Mesozoic". They may have given birth to tiny live neonates rather than laying eggs.

• Fruitafossor, from the Upper Jurassic about 150 mya, was about the size of a chipmunk. Its teeth, forelimbs and back suggest that it broke open the nest of social insects to prey on them (probably termites.
• Spinolestes also had adaptations for digging, so it also might have had anteater-like habits. It is notable for quills like those of modern spiny mice.
• Volaticotherium is from the boundary with the Lower Cretaceous about 125 mya. It is the earliest-known gliding mammal and had a gliding membrane that stretched out between its limbs, rather like a modern flying squirrel. This suggests it was active mainly during the day. It is not the only example of this type of locomotion.
• Repenomamus, from the early Cretaceous 130 mya, was a stocky, badger-like predator that sometimes preyed on young dinosaurs. Two species have been recognized, the largest over a metre long.
• Ichthyoconodon is known from mollariforms found in marine deposits. These teeth are sharp-cusped and similar in shape to those of piscivorous mammals. This has been taken to mean that it was a marine mammal, one of the few examples known from the Mesozoic.

This paints a picture of small-sized mammals being already quite successful and diversified in the Jurassic and early Cretaceous.