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.
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.
Mammaliaformes