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Mammals of the Mesozoic: The least mammal-like mammals

Mammals are generally hairy creatures, though there are exceptions. Nevertheless, even dolphin hair isn't totally unknown. Mammals try to maintain a constant body temperature, feed their babies on milk, reach a maximum adult size and then stop growing, and replace their teeth only once a life-time, (or get a third set from a dentist), should they bother with teeth.

There are a number of other features which are less well known. All living mammals have a lower jaw made from only one bone, they have a different joint for attaching that jaw to the head than all other existing animals, and the inner ear contains three small bones for detecting sound.

Many other 'obvious' shared characteristics don't quite work. Give birth to live young? A platypus doesn't. Have seven neck bones? Not sloths and manatees. Only the females produce milk? Actually, there's this weird dayak fruit bat in Malaysia...

One of the fun things in paleontology is the way in which even the really shared characteristics often break down. And the further back you look, the more it happens.

A long time ago there used to be a couple of creatures called Sinoconodon and Morganuconodon. They lived at the same time, in the same place, and probably led fairly similar lives. The best remains are known from the Lower Jurassic of China. Those are about 180 million years old and surprisingly well preserved. Sinoconodon has caused paleontologists a lot of headaches. For a thing which is supposed to be a basal mammal, it had some very strange habits.

This animal was first named in the 1960s. Further fossils were then found, but they tended to have annoying differences; there'd be more (or less) teeth than expected; they'd be bigger (or smaller). These were seen as belonging to various species, and several were referred to a different genus.

Eventually, someone noticed peculiar details concerning the teeth. Incisors had been replaced at least three times, and a canine on at least four occasions. No wonder varying numbers were found on different jaws. As well as being a serial tooth replacer, Sinoconodon displayed several other non-mammalian characteristics. As with present day reptiles, this creature only stopped growing when it died. Furthermore, the bones around the mouth in baby specimens aren't very well developed, which suggests they weren't able to suckle. Despite all this, the animal is much more similar to Morganucodon then to anything else, and that's why it's often referred to as a mammal.

Morganucodon suckled, replaced its teeth and reached a maximum size in properly mammalian ways. Like Sinoconodon and all other mammals its inner ear was housed within one bit of bony casing. That's not known for any animals other than mammals. It also had three small bones wired up for detecting sound. However, only one was actually inside the ear. The other two were close but formed part of a multi-boned lower jaw. Both animals had the typical mammal jaw joint, but they also possessed a small, non-mammalian joint as well.

As far as is known, these details applied to virtually all mammals then alive, but no living animals are built like this. The final line of mammalian old-timers were the docodonts. Fossils date back to at least the Middle Jurassic. Later docodonts have been found in North America, Europe and Asia. An oddity of this group is their lower molars. These teeth were complexly constructed and must've been very effective at processing food. What may have stopped these animals being more successful than they were was the emergence of 'advanced' mammals, whose teeth were more efficient still. The latest undisputed docodont was Sibirotherium from the Lower Cretaceous of Eastern Russia. Unsurprisingly, it's from Siberia and was first described in 2003. There may be a later representative from the Upper Cretaceous of South America, but this interpretation is disputed.

With most docodonts, remains are restricted to isolated teeth or scraps of jaw. As yet, there are two exceptions. A partial skeleton of Boreolestes has been reported, (but not described), from the Middle Jurassic of Britain. Fossils of Haldanodon from the Upper Jurassic of Portugal are even better. These include a number of skulls and one reasonably preserved skeleton. The short, broad bone in the upper arm suggests this animal was adapted for digging. It was about mole-sized, but probably wasn't a full-time miner. The anatomy of moles is still more specialized. Given the swampy conditions which then prevailed at the location, (Guimarota), and some similarities to other living mammals, Haldanodon was likely a semi-aquatic resident of river banks.

Fact File

Meaning: 'Chinese tooth'
Place: Yunnan, China
Age: Lower Jurassic
Remarks: Fossils include a number of skulls. A similar, but earlier tooth has been reported from the Upper Triassic of France.

Meaning: 'Morgan's tooth'
Place: Western Europe, China & North America
Age: Upper Triassic - Lower Jurassic
Remarks: Many jaws and other remains have been found in Wales. Finds from Yunnan include skulls and skeleton material

Remarks: Docodonta is an extinct order of mammals characterized by complicated teeth on old-fashioned jaws. They're known from the Middle Jurassic - Lower Cretaceous of the northern hemisphere. A possible representative has been reported from the Upper Cretaceous of Argentina, but its exact identity is presently a matter of discussion.

Meaning: 'Siberian beast'
Place: Kemerovo Region, Siberia
Age: Lower Cretaceous
Remarks: Details of the molars show this is a member of a distinctive Asiatic family of docodonts, which were possible descendants of something like Boreolestes.

Meaning: 'northern robber'
Place: Isle of Skye and Oxfordshire, UK
Age: Middle Jurassic
Remarks: This animal is known from lower jaw and tooth fossils. A partial skeleton has yet to be scientifically described.

Place: Guimarota, Portugal
Age: Upper Jurassic
Remarks: Numerous fossils all represent the same species. These include a couple of hundred jaws, ten skulls and an incomplete skeleton. It's the best preserved docodont.

Trevor Dykes (not a paleontologist), 30.3.2004.

"Do you have more information about Mesozoic mammals on-line?"
Yes. Look here.

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