Tag: theria

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 #27: Treetop Origins

Juramaia

The other major branch of the therian mammals are the eutherians – represented today by only the placentals. But although the first eutherians split from their common ancestor with the metatherians all the way back in the mid-Jurassic, more then 160 million years ago, true placentals don’t seem to have actually appeared until right after the end-Cretaceous mass extinction.

There were many other side-branches of the eutherian lineage during the Mesozoic, however, and these last few days of the month will cover some of them.

(Some of these relationships are still under dispute, such as the exact position of leptictidans and Purgatorius and whether they’re actually placentals or not. Eutherian phylogeny is a bit of a mess.)

Juramaia was the earliest known eutherian, living during the Late Jurassic of China (161-156 mya). About 15cm long (6″), it was a shrew-like insectivore with limb anatomy that would have allowed it to climb up trees in a similar manner to modern rats.

Something very similar to it would have been the common ancestor of all later eutherians – suggesting that the earliest members of the group may have started out as tree-climbers before diversifying into different niches later on.

But despite it being closer related to living placentals than to marsupials, placental-style reproduction hadn’t actually evolved yet and it would have still given birth to tiny undeveloped young.

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.

Month of Mesozoic Mammals #25: Sabertooths

Lotheridium

In modern times the therian mammals are represented by just two surviving groups – placentals and marsupials. But both of these lineages contained many other extinct close relatives, and this final week of the month will focus on a few of them.

Modern marsupials are part of a larger grouping known as metatherians which split off from their common ancestor with placentals during the Jurassic period, at least 160 million years ago. Probably originating in Asia, they spread to Europe and the Americas during the Cretaceous, and diversified into several different groups – including some which would go on to become the dominant mammalian predators in South America during its long period of isolation in the Cenozoic.

One of the earliest branches of the metatherians were the deltatheroideans. These mammals are known from Asia and North America, evolving to fill vacant small carnivore niches after the disappearance of most of the eutriconodonts. Most of them went extinct at the end of the Cretaceous, but a few did survive for at least 10 million years afterwards.

Lotheridium was a deltatheroidean living during the Late Cretaceous of China (72-66 mya), and is known from a nearly complete skull. It’s likely to have had a head-and-body length of about 20cm (8″), and may have had a full length including the tail of up to 30-40cm (1′-1′4″).

It had elongated canine teeth, convergently similar to the saberteeth of many later mammal groups. It was probably a highly specialized predator, and may even have been capable of preying on small dinosaurs – its close relative Deltatheridium is known to have eaten theropods like Archaeornithoides.

Lycopsis longirostrus

Lycopsis longirostrus*, a sparassodont mammal from the Middle Miocene of Colombia, South America (~14-12 mya). About 35cm tall at the shoulder (1′2″), it was originally thought to be a dog-like or thylacine-like animal, but studies of a well-preserved specimen have shown that it had an unusual mix of anatomic features in its limb bones, with adaptations for both climbing and running.

It was probably a terrestrial ambush hunter, preying on rodents and other small animals, but also had the ability to grasp and climb trees in its densely forested environment – especially useful for avoiding the land crocodiles that shared its habitat.

( * Often seen misspelled as L. longirostris )