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 #04: The Famous One

Megazostrodon

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.

Month of Mesozoic Mammals #03: Almost-Mammals

Adelobasileus & Sinoconodon

The exact line between “highly mammal-like cynodonts” and “actual mammals” is very blurry. The transition was gradual and the fossil record is incomplete, and even the definition of “mammal” varies depending on who you ask. Do we take the strictest possible route and only include everything coming after the most recent common ancestor of all living mammals – the “crown group” Mammalia itself? Or do we go broader and also include the closely related Mammaliaformes, which already had some of the defining anatomical features of mammals?

(For the purposes of this theme month I’m considering mammaliaformes to count as mammals, but if you prefer the crown group definition then it’ll be a few more days before we reach Mammalia-proper.)

An illustration of an extinct early mammal. It's a rat-likeor shrew-like animal with a long snout, small ears, and a long furry tail.
Adelobasileus cromptoni

The earliest ancestral mammaliaformes would have looked something like Adelobasileus, a transitional form from the Late Triassic of Texas, USA (221-205 mya). About 10-15cm long (4-6″), it was probably a shrew-like insectivore and may have been close to the start of the hypothetical “nocturnal bottleneck” in mammal evolution – a point where mammal ancestors are thought to have taken up nighttime activity patterns to avoid competition and predation from early dinosaurs.


An illustration of an extinct early mammal. It resembles a mixture of a rat and a weasel, with a long low blunt snout, small ears, short legs, and a long bushy tail.
Sinoconodon rigneyi

Sinoconodon is known from the Early Jurassic of China (196-189 mya). Unlike later mammals it seems to have experienced reptile-like continuous slow growth throughout its lifespan, and had multiple replacements of some of its teeth.

Fossils of several different life stages have been found, averaging at similar sizes to Adelobasileus, but the biggest and longest-lived specimens are estimated to have reached the size of a large brown rat at around 35cm long (1′2″) and 500g in weight (~18oz) – big enough to be a weasel-like carnivore feeding on small vertebrate prey.

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 wellesi skull by 5of7 || CC BY 2.0

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.

Month of Mesozoic Mammals #01: Cynodont Ancestors

Welcome to March, and the Month of Mesozoic Mammals!

Although traditionally depicted as tiny “boring” shrew-like animals completely overshadowed by the dinosaurs they lived alongside, in the last decade or two we’ve discovered that Mesozoic mammals were actually incredibly diverse. They ranged in size from only a few centimeters to over a meter long, adapted to a wide range of ecological niches, and developed into some of the most successful and long-lived mammal groups of all time.

So this month we’ll be looking at how mammals evolved and experimented during the Age of Dinosaurs, from their earliest Triassic ancestors all the way to the end-Cretaceous extinction.

Starting with…


Thrinaxodon & Trucidocynodon

All mammals are synapsids (related to animals like Dimetrodon), and are descended from a group known as the cynodonts.

Cynodonts originated in the Late Permian, about 260 million years ago, and were one of the few synapsid lineages to survive through the devastating Permian-Triassic extinction. Although not quite mammals themselves, their skeletons were already fairly mammal-like, with semi-upright postures, differentiated thoracic and lumbar vertebrae, a secondary palate that allowed them to eat and breathe at the same time, and pits on their snouts suggesting they had well-developed whiskers – which would also imply the presence of a coat of fur, since whiskers are modified hairs.

An illustration of an extinct cynodont, a close relative of early mammals. It's a vaguely badger-like animal with small ears and a long tail, pictured curled up asleep.
Thrinaxodon liorhinus

Thrinaxodon was an early cynodont about 50cm long (1′8″), living in the Early Triassic of South Africa and Antarctica shortly after the mass extinction (~252-247 mya).

It was capable of digging, with complete specimens found curled up inside their burrows, including pairs that may indicate some form of social behavior and one instance of sharing with a temnospondyl amphibian.


An illustration of an extinct cynodont, a relative of early mammals. It's a somewhat badger-like animal with small ears and a long tail.
Trucidocynodon riograndensis

Trucidocynodon lived later during the Triassic in Brazil (~220 mya) and was one of the biggest known non-mammalian cynodonts at around 1.2m long (4′).

It had more upright limbs than some of its other relatives, and a semi-digitigrade stance that seems to have been adapted for running, suggesting it was an active predator. Considering it was living at a time when predatory crocodile-relatives and early dinosaurs were thought to be the dominant carnivores, its large size is especially surprising.

Unsolved Paleo Mysteries Month #23 – Puzzling Proto-Bats

Let’s finish off this month the same way we started: with flying vertebrates without any transitional forms!

Much like the pterosaurs, bats appear suddenly in the fossil record already fully flight-adapted. Despite being the second-largest group of mammals, bats’ small fragile bones and terrestrial habitats make fossils of them incredibly rare, and transitional forms are still entirely unknown. (Even the ancestral form illustrated above is a generic hypothetical mammal!)

The most “primitive” known bats come from the Early Eocene* (~55-52 mya) and various early representatives have been found as far apart as North America, Europe, India, and Australia – indicating they were already a widespread and diverse group by that time, and making it difficult to pin down just where and when they actually might have originated.

*I’ve seen mentions of a potential bat-like tooth from the Late Cretaceous of South America, but can’t find any actual references for it. So it’s possible bats may even have evolved before the K-Pg extinction.

Although bats were once thought to be related to archontans (treeshrews, colugos, and primates) based on morphological similarities, more recent genetic studies have shown them to instead be grouped with the laurasiatheres (eulipotyphylans, carnivorans, pangolins, ungulates, and whales). Based on this phylogenetic position the earliest ancestors of bats may have been small tree-climbing shrew-like animals who evolved flight while leaping in pursuit of insects. They might even be closely related to an obscure group called nyctitheriids – but without a lucky find of an exceptional fossil, we just don’t know.

Unsolved Paleo Mysteries Month #22 – Gargantuan Godzillus

During the summer of 2011 amateur paleontologist Ron Fine discovered an unusual fossil in northern Kentucky, USA. Beginning with an oddly-textured nodule, he gradually excavated a structure about 1m wide and 2m long (3′3″ by 6′6″) – far larger than any other fossil found in the area, and unlike anything seen before.

Dating to the Late Ordovician (~450 mya), and nicknamed “Godzillus” due to its massive size, the strange fossil has a roughly elliptical shape with what appear to be multiple branching lobes, and a surface texture made up of many rows of tiny bumpy “scales”. In some places small trilobites are found directly attached to it, most likely either feeding on it or sheltering.

The full “Godzillus” fossil and a close-up of its complex surface texture [Image source]

Although a paper has been published on Godzillus, it’s been given no official scientific name or classification. At best it’s been called an “organically textured surface”, with two different main possibilities suggested for its identity – either an algal mat that was deformed by water currents and smothered by a turbidity flow, or some sort of unknown upright soft-bodied organism that was toppled over and similarly buried.

Both hypotheses have their own strengths and weaknesses, and each explain some features of the structure but not others. So, for now, Godzillus remains a bizarre and unique fossil.

Unsolved Paleo Mysteries Month #21 – Ancient Aquatic Aliens

Found only in the Carboniferous-aged Bear Gulch Limestone (~318 mya) in Montana, USA, Typhloesus wellsi is such a confusing animal that it’s been nicknamed “the alien goldfish”.

It was one of the first body fossils found containing conodont elements, leading to it initially being identified in the 1970s as the then-unknown conodont animal – until actual conodont animals were discovered a few years later, looking nothing like it. The elements were reassessed as actually being Typhloesus’ gut contents, indicating it was actually a conodont-eating predator or scavenger.

Reaching sizes of almost 10cm long (4″), it was vaguely fish-shaped with a pair of ventral fin folds and a stiffened vertical tail paddle. No obvious sensory structures are preserved, but there are impressions of a large gut cavity in the front half of its body, along with a pair of strange unidentified organs known as “ferrodiscus” that contained a high concentration of iron deposits.

And despite being known from over 50 specimens, we still don’t know where to classify it. At all. It lacks evidence of features like gill openings or a notochord that could associate it with chordates. Its gut appears to be a blind sack with no anus, a condition usually seen only in cnidarians and flatworms, and finned active swimmers are known in other invertebrate groups like molluscs and arrow worms, but Typhloesus doesn’t resemble anything like those either.

With the similarly mysterious Tullimonstrum recently getting a lot of attention and a possible identification as a lamprey-relative, perhaps somebody will eventually have another look at this strange little creature, too.

[EDIT: A 2022 study found evidence of a molluscan affinity for Typhloesus!]

Unsolved Paleo Mysteries Month #20 – Dubious Diskagma

Only around 0.3-1.8mm long (0.01-0.7″), Diskagma buttonii is known from the Paleoproterozoic of South Africa, dating to a whopping 2.2 billion years ago. These tiny urn-shaped structures were connected into bunches, and were found in rocks that were once ancient soils – hinting that there may have been some form of (relatively) complex life present on dry land much earlier than previously thought, as far back as the early stages of the Great Oxygenation Event.

But what these things actually were is a mystery. Their size and complexity resemble some sort of eukaryote, which would make them one of the oldest known representatives. They also have some similarities to the older fossil Thucomyces lichenoides, the younger fossil Horodyskia, and a modern fungus with internal symbiotic cyanobacteria.

Or they could be a very early “experimental” branch of life with no close living relatives. For now, we just don’t know.