convergent evolution – Page 10

Month of Mesozoic Mammals #11: Getting Airborne

Volaticotherium

One of the earliest major branches of the theriiformes were the eutriconodonts. First appearing in the fossil record in the Early Jurassic, about 190 million years ago, these mammals were a highly successful group that adapted to a variety of different niches and lasted up until nearly the end of the Cretaceous.

Their exact relationships to other theriiformes are a little uncertain, with it being unclear whether they split off before or after the multituberculates (another major group featured later this month).

Volaticotherium is known from the Middle Jurassic of China (165-161 mya), and was the first gliding Mesozoic mammal to be discovered (although we now know about quite a few more). It was part of a branch of the eutriconodonts known as the volaticotherians, a widespread lineage which ranged through most of the Jurassic period and into the mid-Cretaceous.

Measuring about 26cm long (10″), or about 14cm (5.5″) excluding the tail, it’s known from a mostly complete skeleton with impressions of fur and skin. A gliding membrane extended from its hands to its hindlimbs and the base of its tail, its feet had grasping toes, and its tail was flattened to create an airfoil-like shape.

It had sharp slicing teeth, indicating a carnivorous or insectivorous diet – unusual since most other known gliding mammals are predominantly herbivores.

Month of Mesozoic Mammals #10: Termite-Eaters

Fruitafossor

Living during the Late Jurassic of Colorado, USA (156-150 mya), Fruitafossor was one of the earliest known mammals specialized for feeding on colonial insects. It had peg-like enamel-less teeth and a reinforced spine surprisingly similar to those of modern armadillos and anteaters, and powerful digging forelimbs with only four fingers on each hand.

It’s known from an almost complete skeleton, about 15cm long (6″), but its highly modified features make figuring out its exact evolutionary relationships rather difficult. It may have been part of a very early offshoot of the theriimorph lineage, something with no close living relatives but still converging on the exact same adaptations as placental mammal groups that wouldn’t emerge until the Cenozoic 100 million years later.

Month of Mesozoic Mammals #07: Digging Down

Docofossor

The docodonts didn’t stop at exploiting ecological niches in the trees and water. Another branch of the group specialized into underground burrowing, developing convergent features remarkably similar to modern golden moles.

Docofossor is known from the Middle Jurassic of China (161-155 mya), and measured about 10cm long (4″). It had large shovel-like fingers, strong forelimbs, short sprawling hindlimbs, and pointed teeth adapted for capturing invertebrate prey. (I’ve also given it a patch of protective keratinized skin on its snout here, based on the related Haldanodon.)

It had a reduced number of bones in its fingers, a modification identical to some modern mammals – suggesting that these relatively “primitive” mammals were already using the exact same genes to regulate their anatomical development.

Month of Mesozoic Mammals #06: Going for a Swim

Castorocauda

While some docodonts like Agilodocodon were going up into the trees, another branch of the group was specializing into semi-aquatic habits instead.

Castorocauda is known from the Middle Jurassic of China (165-161 mya), represented by an exceptionally preserved fossil showing soft tissue and hair impressions. About 40cm long (1′4″), it would have lived in a wetland environment and was well-adapted for swimming, with a flattened scaly beaver-like tail, webbed toes, and a coat of dense fur very similar to that of modern mammals, made up of both guard hairs and underfur.

Its strong forelimbs suggest it was capable of digging burrows, like modern platypus, and its sharp backwards-pointing teeth indicate a diet of slippery prey such as fish and worms.

It was also one of the earliest known mammals with (possibly venomous) spurs on its ankles. This feature is only seen today in monotremes, but seems to have been an ancestral trait common to all early mammals that was later lost in the lineage leading to marsupials and placentals.

Month of Mesozoic Mammals #05: Climbing Trees

Agilodocodon

Before we get to the actual-Mammalia-mammals, there’s one more group of mammaliaformes who deserve some attention – the docodonts.

Falling evolutionarily just outside of Mammalia itself, docodonts first appeared in the mid-Jurassic and lasted until the Early Cretaceous. They used to only be known from teeth and jaw fragments and were thought to have been fairly generic shrew-like terrestrial insectivores, but more recent discoveries have shown them to have actually been some of the earliest mammals to specialize into diverse habitats.

Agilodocodon was adapted for climbing around in trees, making it one of the earliest known arboreal mammals (although not the first climbing synapsid). Living in China during the Middle Jurassic (165-161 mya), it measured about 13cm long (5″) and had sharp gripping claws and flexible wrists and ankles similar to modern climbing mammals like tree squirrels.

When it was first described in 2015 it was suggested that its spade-like front teeth were specialized for gnawing bark and feeding on tree sap – but a later study found that its teeth didn’t really resemble those of any modern sap-eating mammals, and in fact were closer in shape to those of insectivorous marsupials and elephant shrews.

Month of Mesozoic Mammals #02: Swimming Cousins

Kayentatherium

Known from the Early Jurassic of Arizona (196-183 mya), Kayentatherium was part of a group of cynodonts called tritylodontids – very close cousins of the true mammals, specialized for herbivory. They had strong jaw muscles, large incisors, and grinding cheek teeth, an arrangement convergently similar to modern rodents, and were some of the latest-surviving non-mammalian synapsids, persisting into the Early Cretaceous.

Kayentatherium was one of the larger tritylodontids at just over 1m long (3′3″), and appears to have been semi-aquatic, with oar-shaped hindlimbs and a flattened beaver-like tail. Although not the first non-mammalian synapsid to be interpreted as a swimmer, it was the earliest close relative of the true mammals to develop these sorts of adaptations.

Thoatherium

While this animal might look like some sort of deer or horse, it was actually only distantly related to any modern hoofed mammals.

This is Thoatherium from the Early Miocene (~17-16 mya) of Argentina. About 70cm long (2′3″), it was related to the weird llama-like Macrauchenia and was part of an extinct group of ungulates (the Meridiungulata) which evolved during South America’s time as an isolated island continent.

It was adapted for fast running, with long legs and only a single horse-like hoof on each foot – but it was even more one-toed than modern horses are, having no remaining “splint bones” from vestigial side toes.

Shringasaurus

Shringasaurus indicus, an archosauromorph reptile from the Middle Triassic of India (~247-242 mya). About 3-4m long (9′10″-13′1″), it was part of a group known as the allokotosaurs, specialized herbivores with lizard-like bodies, and was related to the strange long-beaked Teraterpeton.

Partial remains of about seven different Shringasaurus individuals of varying ages have been found, with several of them having large curving horns over their eyes convergently similar to those of the later ceratopsids. A couple of adult specimens lack horns entirely, suggesting the feature was sexually dimorphic with only the males developing ornamentation – a very rare arrangement among archosauromorphs, but similar to some horned mammals.

Potanichthys

Potanichthys xingyiensis, a fish from the Middle Triassic of China, living around 235-242 million years ago.Measuring about 15cm long (6″), it was one of the oldest known fish capable of aerial gliding – possessing a “four-winged” body plan with enlarged pectoral and pelvic fins, and an asymmetrical tail with a long lower lobe. It was also almost completely scale-less, which may have helped to reduce drag and make it more aerodynamic.

Despite the similar appearance it had no close relation to modern flyingfishes, and was instead a result of convergent evolution in a completely different lineage of the ray-finned fishes.

Bathornis

Bathornis grallator, a flightless bird about 75cm tall (2′6″) from the Late Eocene and Early Oligocene of Midwestern USA (~37-34 mya).

It was originally mistaken for a long-legged vulture (under the name Neocathartes) when first discovered in the 1940s, but later studies have shown it was actually one of the smaller members of the bathornithids – close cousins of the more well-known South American “terror birds”, successfully occupying terrestrial predator niches alongside large carnivorous mammals.