Tag: convergent evolution

Almost-Living Fossils Month #19 – Even More Metatherians

While the opossum-like herpetotheriids and peradectids survived in the northern continents for most of the Cenozoic, a wider variety of metatherian mammals were found in the south. Alongside the true marsupials and the sparassodonts, a group known as the polydolopimorphs existed in South America for over 60 million years. Although most of the their fossil remains consist only of isolated teeth and jaw fragments, they seem to have been a very diverse group that adapted to a wide range of ecological niches including insectivores, herbivores, and fruit-eating and seed-eating specialists.

Their exact evolutionary position within the metatherians is still rather unclear and under dispute, with different studies giving different results. They were probably marsupialiformes, slightly less closely related to marsupials than the herpetotheriids, but some paleontologists instead consider them to have been true marsupials related to either the shrew opossums or the microbiotheres. (And some go with both options, proposing that they weren’t even a natural group but were polyphyletic, with some being marsupialiformes and others being true marsupials.)

The earliest definite polydolopimorph fossils come form the very start of the Paleocene in South America (~66 mya), but their lineage likely goes further back into the Late Cretaceous – possible remains from North America suggest they may have originated there at least 70 million years ago, with their ancestors migrating into South America shortly before the end-Cretaceous extinction. A few also reached Antarctica by the Late Eocene (~40-33 mya), before the continent had fully separated from its neighbors and frozen over, but it’s unclear whether any ever made it as far as Australia alongside their marsupial relatives.

They were most diverse during the first half of the Cenozoic, and in the latter half they were represented mainly by a highly specialized lineage called the argyrolagids. Known from western and southern South America (Peru, Bolivia, Argentina, and Chile) from the Early Oligocene onwards, these polydolopimorphs were convergently rodent-like desert herbivores with short forelimbs and long hopping hindlimbs that gave them a resemblance to jerboas or springhares.

Argyrolagus palmeri here lived during the Early Pliocene of Argentina (~5-3.5 mya). About 40cm long (1′4″), it had only two toes on its feet, a long pointed snout, and large eyes and ears that indicate it was probably nocturnal.

These last polydolopimorphs survived until at least the end of the Pliocene, around 2.5 million years ago. Their disappearance coincides with the time of the Great American Interchange – when South America became connected to Central and North America – and they may have been some of the victims of the extinction caused by the influx of placentals from the north.

Almost-Living Fossils Month #15 – Digging Dryolestoids

First appearing in the mid-Jurassic, about 168 million years ago, a group of mammals called dryolestoids were some of the closest known relatives to the therians (the group that contains modern marsupials and placentals). They were found throughout North America, Eurasia, and North Africa up until the Early Cretaceous (~125 mya), but then mostly disappeared from the northern continents and instead migrated into South America – where they went on to flourish and became some of the most common mammals in the continent by the Late Cretaceous.

Although mostly known from only teeth and jaw fragments, the dryolestoids seem to have been a pretty diverse group of mammals during the Mesozoic, adapting to a variety of different diets and lifestyles ranging from small insectivores to relatively large herbivores.

Most of them died out in the end-Cretaceous mass extinction, except for the dog-sized herbivorous Peligrotherium which survived until 59 million years ago in the Early Paleocene… and one other known example from much much later into the Cenozoic.

Despite being absent from the fossil record for over 40 million years, the dryolestoids reappeared again in the Early Miocene of Patagonia (~21-17.5 mya) with a single late-surviving member: Necrolestes patagonensis.

The known Necrolestes fossils are surprisingly well-preserved compared to most other dryolestoids, with about a third of its skeleton represented. It was a small mole-like burrowing animal, about 10-15cm long (4-6″), with large canine teeth and an upturned snout. The cartilage in its nose was ossified into bone, strengthening it and probably supporting a pad of thick toughened skin – and also suggesting that it was a “head-lift digger”, using its snout like a shovel to dig through the soil.

While it superficially resembled the earlier mole-like dryolestoid Paurodon, it was actually much closer related to more generalist Mesozoic forms like the sabertoothed Cronopio.

After Necrolestes there’s no further evidence of dryolestoids living any closer to modern day. Much like the late-surviving gondwanatheres they lived alongside, these last dryolestoids may have specialized themselves so much that they couldn’t cope with sudden environmental changes, and the Middle Miocene extinction could have finished them off entirely.

Almost-Living Fossils Month #13 – Some Long Salamanders

A group of salamanders called batrachosauroidids first appeared in the fossil record at the very end of the Jurassic, about 145 million years ago, originating in Europe and quickly spreading to North America. Long snake-like bodies and reduced vestigial limbs gave these amphibians a very similar appearance to modern amphiumas or sirens, but they weren’t actually very closely related to each other – instead, the batrachosauroidids’ closest living relatives are thought to be mudpuppies and the blind cave-dwelling olm.

They were probably fully aquatic, living in wetlands with slow-moving currents, and the structure of their jaws suggest they were active predators that would have fed on other smaller animals in the water.

After surviving the end-Cretaceous mass extinction fairly well, with fossils of a couple of species known from both sides of the Cretaceous-Paleogene boundary, the batrachosauroidids continued on for most of the Cenozoic. They seem to have disappeared from Europe first, with the last known record in the mid-Eocene (~40 mya), but they persisted in North America for at least another 25 million years, well into the Miocene.

One of the last known members of the group was Batrachosauroides dissimulans from the mid-Miocene of Florida and Texas, USA (~16-13 mya). It was also one of the largest of the batrachosauroidids, similar in size to modern amphiumas at around 1m long (3′3″).

Past that point in time there’s no further evidence of batrachosauroidids, although due to the rather poor fossil record of salamanders it’s possible they may have survived for a while longer – but since amphiumas and sirens began to develop larger body sizes from the Late Miocene onwards, it’s likely that they were evolving to fill the ecological niches left vacant by the extinction of the last batrachosauroidids.

Copepteryx

Copepteryx hexeris, a plotopterid bird from the Late Oligocene of Japan (~28-23 mya).

Known from around the North Pacific rim from about 33-15 million years ago, plotopterids were flightless diving birds which used their small but powerful wings to propel themselves through the water. They were convergently similar to penguins in body shape and lifestyle, but not actually closely related to them – instead being relatives of gannets, cormorants, and anhingas.

Smaller plotopterids were about the size of modern cormorants, around 70cm long (2′4″), but the larger known genera like Copepteryx rivalled the southern giant penguins at around 1.8m (6′).

And a second species of Copepteryx known only from a single leg bone (Copepteryx titan) may have been ever bigger. Estimated at over 2m in length (6′6″), it was possibly one of the largest diving birds to have ever lived.

Procynosuchus

Procynosuchus delaharpeae, a synapsid from the Late Permian (~259-252 mya). Measuring about 60cm long (2′), it was one of the earliest members of the cynodonts, the lineage that would eventually lead to mammals.

Its fossils are mostly known from southern Africa, but similar remains have also been found in Europe and Russia, suggesting it was actually quite widespread across the supercontinent of Pangaea that existed at the time.

It had a long vertically-flattened tail, strong leg muscles, and paddle-like feet – all adaptations that suggest it was a semi-aquatic otter-like animal capable of agile swimming. It also had forward-facing eyes, giving it good binocular vision and depth perception while pursuing fish underwater.

Nicrosaurus

Nicrosaurus kapffi from the Late Triassic of Germany, about 221-205 million years ago. Although rather crocodile-like in appearance, this 4-6m long (13′-19′8″) animal was actually part of an extinct group called phytosaurs – long-snouted heavily-armored reptiles with their nostrils high up on their heads near their eyes.

Phytosaurs’ exact evolutionary relationships are still disputed, with opinions currently going back and forth between them being archosauriformes or an early branch of the croc lineage within the true archosaurs. But either way they weren’t directly ancestral to modern crocodilians, and instead developed a very similar body plan via convergent evolution.

While some phytosaurs had very slender gharial-like snouts and probably fed mostly on fish, others like Nicrosaurus had much more robust jaws and seem to have secondarily adapted to a terrestrial predator lifestyle. They had longer limbs and a more upright posture than their semi-aquatic relatives, and enlarged fangs at the hooked tips of their jaws that may have been used to deliver a powerful stabbing blow to their prey.

Nicrosaurus also had a raised bony crest running along its snout, which I’ve depicted here as supporting an even larger soft-tissue display structure.

Eudibamus

Eudibamus cursoris, a bolosaurid from the Early Permian of Germany (~284-279 mya).

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

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 convergently ostrich-like feet with only two toes each.

Month of Mesozoic Mammals #28: Hop To It

Zalambdalestes

Living during the Late Cretaceous of Mongolia (85-70 mya), Zalambdalestes was part of a highly specialized group of mammals that it lends it name to – the zalambdalestids – which were an early branch of the eutherian evolutionary tree.

About 20-25cm in length (8-10″), it had relatively long limbs with especially strong hindlegs that show adaptations for rabbit-like hopping. Its long narrow snout may have ended in a flexible proboscis similar to those of modern sengi, and sharp interlocking teeth indicate a carnivorous or insectivorous diet.

Its long rodent-like incisors grew continuously throughout its life, suggesting it was gnawing on something tough enough to constantly wear down its front teeth.

Skull of Zalambdalestes || from fig 51 in Wible JR, Novacek MJ, Rougier GW (2004) New data on the skull and dentition in the Mongolian late Cretaceous eutherian mammal Zalambdalestes. Bulletin Of The American Museum Of Natural History 281:1-144 uri: http://hdl.handle.net/2246/449

Some studies have proposed that zalambdalestids were actually very basal members of placental mammal groups such as rodents or rabbits, but the presence of epipubic bones in front of their pelvises (bones not found in placentals) shows they were a much earlier type of eutherian that still reproduced more like marsupials. Any anatomical similarities to later placentals were probably just the result of convergent evolution.

Month of Mesozoic Mammals #26: Crunch Time

Didelphodon

While true marsupials didn’t appear in the fossil record until the early Cenozoic, some of their closest relatives in the Mesozoic were a group called stagodontids. These metatherians evolved in North America during the Late Cretaceous, and much like the deltatheroideans seem to have taken over some of the ecological niches left vacant after the extinction of most eutriconodonts.

Their strong jaws and large blunt premolar teeth were adapted for crushing hard-shelled food, and they seem to have been semi-aquatic swimming animals (somewhat similar to modern water opossums) specialized for eating freshwater invertebrates such as snails and crustaceans.

Most stagodontids went extinct during the end-Cretaceous mass extinction, but if the early Cenozoic genus Eobrasilia is a member of the group then at least some survived in South America up until about 52 million years ago.

Other metatherians persisted in Europe, Africa, and Asia for most of the Cenozoic, with some fossils dating to just 11 million years ago, and the sparassodonts were successful in South America until about 3 million years ago – but today the only living members of this branch of the therian lineage are the marsupials in the Americas and Australasia.

Didelphodon was a stagodontid living during the Late Cretaceous of North America (70-66 mya), and is known from skull and jaw remains, along with a fairly complete skeleton that hasn’t been officially described yet.

It had a long otter-like body and flexible feet, adaptations for efficient swimming, and was one of the largest known mammals at the time, measuring around 1m in length (3′3″).

Its sturdy jaws had an especially powerful bite force, one of the strongest relative to body size known for any mammal, and some of its molars were blade-like and similar in appearance to carnassials. Along with crunching on shellfish it would also have been capable of crushing bones and tough plants, and its diet was probably an omnivorous mixture of invertebrates, vegetation, carrion, and small vertebrates – potentially including other mammals and small dinosaurs.