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
Sinoconodon
Morganucodon
Docodonts
Sibirotherium
Boreolestes
Haldanodon
Trevor Dykes (not a paleontologist), 30.3.2004.
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