Capinatator praetermissus, an arrow worm from the Mid-Cambrian of Canada (~508 mya). Discovered in the famous Burgess Shale fossil deposits, it was one of the earliest known arrow worms and also much larger than most modern forms, measuring around 10cm in length (4″).
Its mouth was surrounded by 50 hooked spines, which could be extended out to grasp onto its prey – probably feeding on whatever smaller animals it could catch – but when not in use these spines would have been kept folded up inside a fleshy “hood” around its head.
It may have been a transitional form between early large-predator arrow worms and the smaller plankton-feeders that the group later became.
Australovenator wintonensis, a megaraptoran dinosaur from the Late Cretaceous of Queensland, Australia (~100-94 mya). It was a medium-sized member of the group, about 6m long (19′8″), and despite only being known from a partial skeleton it’s still one the best-known megaraptorans – and also the most complete predatory dinosaur from Australia.
Megaraptorans were a group of fairly large theropod dinosaurs, currently known from Australia, South America, and Japan (and maybe Egypt). Their relationships to other theropod groups are rather uncertain, with different studies placing them as neovenatorids, tyrannosaurids, or most recently as an early branch of the coelurosaurs.
They had very lightly-built bodies, with bird-like bones full of weight-reducing air spaces, proportionally small heads with long slender snouts, and leg bones adapted for running. But their most distinctive feature was their hands, featuring massively enlarged claws on the first and second fingers, with the third finger being much smaller and somewhat vestigial-looking. While some other theropods like allosaurids and spinosaurids also had big hand claws, megaraptorans’ almost tyrannosaurid-like mostly-two-fingered arrangement is rather odd.
Their arms and fingers were much more flexible than those of most other non-avian dinosaurs, allowing them to reach out, grab onto prey with those claws, and then pull it in close to their bodies, restraining it in a sort of death-hug while their relatively weak jaws finished it off.
A distinctive injury to the second toe of Australovenator also suggests these dinosaurs may have been able to deliver powerful kicks like modern cassowaries.
Although very lizard-like in appearance, this animal was actually part of a completely extinct group known as parareptiles – a diverse group of early sauropsids who were once thought to be the ancestors of turtles, but are now considered to instead be the evolutionary cousins to the true reptiles.
With a total length of about 25cm long (8-10″), the structure and proportions of its limbs suggest it could run fast on its hind legs, making it one of the earliest known examples of bipedal locomotion. Since its teeth were adapted for a herbivorous diet, it wasn’t using its speed to chase down prey but was instead probably sprinting away from predators.
But unlike the sprawling running of some modern lizards, Eudibamus may have been capable of holding its legs in a more upright position directly under its body, convergently evolving a more energy-efficient posture similar to that of later bipedal animals like dinosaurs.
Ergilornis rapidus, a 1.2-1.5m tall bird (4′-5′) from the Early Oligocene of Mongolia (~33-28 mya). Closely related to modern cranes, trumpeters, and limpkins, it was part of an extinct group called eogruids – flightless birds which existed across Eurasia for a large portion of the Cenozoic from roughly 40-3 million years ago.
Although the earliest known eogruids were smaller and less specialized, and may even have still been somewhat capable of flying, later forms like Ergilornis had highly reduced wings, long legs adapted for running, and convergentlyostrich-like feet with only two toes each.
Known from several near-complete fossils that include rare soft tissue impressions, it’s the first varanopid to show preserved skin details – revealing a pattern of very lizard-like rectangular scales. If it is a synapsid this is a pretty big deal, since early synapsids were previously thought to have had scale-less leathery skin.
It also had unusual mosaic-like patches of tiny osteoderms above its eyes, a feature previously known only in some temnospondyl amphibians. Whether this was the result of convergent evolution or the trait actually being ancestral to most tetrapods is unclear.
Its slender body, long digits, and highly curved claws indicate it was an agile climber. It probably mainly lived up in the treetops, feeding on insects, making it one of the earliest known tetrapods specialized for an arboreal lifestyle.
(*Maybe. There’s apparently an upcoming study that suggests varanopids might actually be sauropsids instead.)
Its short toothless snout suggests it was a suction feeder, using water pressure differences to pull small soft-bodied prey straight into its mouth like a syringe. Along with a heavily built body similar to those of hupehsuchians, and a very long tail that made up over half of its 1.6m length (5′3″), it was probably a fairly slow swimmer living in shallow coastal waters.
It was a close relative of Cartorhynchus, and may have been similarly capable of hauling itself onto land like a modern pinniped.
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.
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.
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.
If there’s one Mesozoic mammal that’s been relatively well-mentioned in dinosaur books and popular media for many years, it’s undoubtedly Megazostrodon. Often depicted as “the first mammal”, it actually occupies a point in the mammal evolutionary tree somewhere between the earliest mammaliaformes and the common ancestor of all modern groups.
Megazostrodon lived during the very end of the Triassic and the Early Jurassic of South Africa (201-189 mya), and is represented by some near-complete fossil material – a rarity for this sort of small ancient mammal, most of which are only known from teeth and other fragments.
About the size of a mouse, only about 12cm long (5″), it was an insectivore with teeth adapted for chewing and crunching through hard arthropod shells. Enlarged regions of its brain associated with the senses of hearing and smell show it was likely nocturnal, occupying an ecological niche similar to modern shrews.
It probably reproduced similarly to modern monotremes, laying small parchment-shelled eggs and lactating from patches of skin. Fossils of the closely related and similar-looking Morganacudon show evidence of toothless infants and juveniles with a single set of milk teeth, suggesting these were some of the first mammals whose young were entirely dependent on milk during the earliest stages of life.