| The Evolution of Mesozoic Mammals, a Rough
Sketch
PLEASE NOTE: THIS PROJECT IS NOT SCIENTIFIC. IT IS A HOBBY.
"I was looking for the feature on the evolution of breasts and lactation."
The Evolution of Mesozoic Mammals, a Rough Sketch
Mammals evolved from cynodont protomammals and made their first appearance during the
Upper Triassic. Crown-group mammals are known
from the Middle Jurassic. The earliest identified representatives of the three existing
lines, (eutherians -including placentals,
metatherians -including marsupials, and the
egg-laying monotremes), are all from the Lower
Cretaceous.
What follows are my own unqualified ramblings. Treat with caution.
I thought I'd try to produce a narrative overview of my on-line directories, and explain a
bit about the reasoning behind the structure.
Mesozoic Eucynodonts, Genus Index:
As the structure has much to do with the present understanding of evolutionary development,
(none of which is in any way my work), this also provides an opportunity to draw up some
sort of sketch of that too, as far as I'm able. There are many gaps in the fossil record.
Nevertheless, the broad sweep of much anatomical development is surprisingly well evidenced.
Of course, new fossils and methods of study will change and deepen understanding as they
become available. As far as possible, I'll attempt to use reasonably accessible English,
though there will be some paleo-tongue-twisters.
BEFORE THE MESOZOIC PLUS MUCH OF THE TRIASSIC
Crudely put, mammals evolved from reptiles, which evolved from amphibians. That's a bit too
crude, and I prefer to think of the first 'reptiles' as pre-reptiles. They also get called
stem-reptiles and many more complicated things besides. That seems to be the lineage from
which 'mammal-like reptiles', known as synapsids, emerged. This development probably
occurred approximately 300 million years ago during the Upper Carboniferous. These
synapsids flourished during the following Permian period, and then seem to have been sliced
off in their prime during a mass extinction. Only three lines are known to have crept into
the Triassic.
The physically smallest of these animals, resplendent with the name of therocephalians,
aren't known beyond the Middle Triassic. The plant eating 'two-dog-teethers', the
dicynodonts, disappeared during the lower Upper Triassic. The third group, the cynodonts,
seem to have mostly gone with them. Some of these were the dominant predators of their days;
a role which was assumed by new fangled things called dinosaurs. There were also other
representatives with herbivorial habits.
However, some of the more derived cynodonts, the eucynodonts,
('true dog tooth'), held on. Rather like Tom Jones in the late 1990s, they even enjoyed a
further flourish to their career. And they're still singing Sex Bomb now.
EUCYNODONTIA (THE 'ADVANCED' CYNODONTS)
The first widely recognized member of this pack is Cynognathus, ('dog jaw'). It's
known mainly from the Lower Triassic, though some finds may pre-date this. It was the top
dog of its time; a wolf-sized predator with pretty much a worldwide range. As befitting
the first acknowledged eucynodont, it's kennelled on my opening directory. It enjoys
the company of various colleagues, which are mostly hard to place more accurately.
In the same subdirectory can be found the remaining, non-mammalian Triassic lineages. I'm
not confident of the validity of the scheme I'm presently using, but it seems to be the best
I can do for now. It involves a clear distinction between herbivore and carnivore lines,
which is far neater than any such arrangement today. Then again, this has to do with
anatomy rather than with the actual diet. The giant panda is clearly a carnivore, despite
its love of bamboozling. Our ancestry lies amongst the meat-eaters, anchored somewhere
within the strangely named superfamily, Chiniquodontoidea. In an effort to be more precise,
the dromatheriids can't be ruled out as culprits. However, this is perhaps because they're
so poorly known. Most genera are based on isolated, very mammal-like teeth.
Triassic Eucynodonts
INTO THE JURASSIC
At least three groups of small to tiny eucynodonts survived into the Jurassic. Enough
remains have been recovered to show all these critters were closely related, although this
cuts across the traditional classes of Reptilia and Mammalia,
if both are defined widely.
I refer to one as 'Jurassic cynodonts'. If taken too literally, this title could be
misleading. Not all the listed creatures were Jurassic, and all eucynodonts are cynodonts.
However, a fair number of people, (eg. the viewers of the first episode of Walking with
Dinosaurs), have at least heard the word cynodonts, although it might not spring readily to
mind. They were those things in the burrow being annoyed by the predatory dinos. I'm
cheating slightly and assuming people will accept this usage as indicatiing nearly-mammals.
It's home to the plant-munching tritylodontids.
According to some views, tritys are descendants of traversodonts. However, the prevailing
opinion now sees them as specialised probainognathians, and close relatives of mammals.
As of April 2005, a seperate directory was established for titheledontans; a loose grouping
of insectivores which just reached the Jurassic. They could be even closer to mammals. As
their lifestyle was so similar to the earliest mammals, trithes may have become obsolete.
The tritys -the herbivores- enjoyed better luck. They've even been reported from the
upper Lower Cretaceous of Siberia and Japan.
Jurassic Cynodonts; Tritylodontidae
Tritheledonta
The third lineage I mentioned is a family called Morganucodontidae. (Ultimately, the name
derives from Mr Morgan and a tooth.) Sticking to a broad definition, these represent the
first known mammals. A little anatomy:
The best known member of this family is Morganucodon. It happens to be remarkably
well preserved, and the following is clear:
It will never be possible to say with certainty that this is the big mama of all mammals,
(for goodness sakes, it was only about 10 cm long). However, it does an extremely good
impersonation of being just that.
Both in terms of time and geography, Morganucodon was a wide ranging genus. Remains
have been reported from the Upper Triassic to the Lower Jurassic, and from across the
northern hemisphere. The best specimens have been recovered from the Jurassic of China.
Before the excess rock was cleaned away, one Chinese fossil was tentatively referred to as
a baby. This turned out to be tantalizingly incorrect. It's the skull of a tiny adult, (or
near adult), which is now known as Hadrocodium. This creature weighed about the same
as a non-Mesozoic paperclip. In terms of anatomy, it's also considerably further along the
evolutionary road towards our selves. Due to a lack of much to compare it with, a better
understanding of this midget must await further discoveries. I've placed it on this
directory for want of anywhere more appropriate.
A family with the somewhat frightening name of Megazostrodontidae might be somewhere near
to the docodonts, though most opinions see closer relationships with the morganucodontids.
Basal Mammaliaformes, Morganucodontidae, Megazostrodontidae and Hadrocodium
EARLY 'SYMMETRODONTS' AND OTHER STRANGE WORDS
The next three directories are rather isolated affairs. There are clues about affinities
with other groupings, though decisive evidence is lacking.
Kuehneotherium and friends
As well as being difficult to spell, the kuehneotheriids are sometimes termed 'obtuse angle
symmetrodonts', which has to do with the configuration of these cusps. They're known only
from bits of jaw and teeth, which hale from the Upper Triassic of Western Europe and the
Lower Jurassic of India and Wales. The jaw construction is of a 'primitive' grade. There
were later 'obtuse angle symmetrodonts' too, but there's little evidence to suggest this is
a sign of kinship. The "amphiodontids" fall into this category. They're poorly
known and of uncertain affinities. The various genera probably aren't even related to
each other.
Kuehneotheriidae and Co and "Amphiodontidae"
Docodonts plus
The modest woutersiids are only known from Upper Triassic teeth from Western Europe.
Docodont teeth are rather distinctive to those who appreciate such things, and the
relationship could be near to an ancestral one, though this is increasingly seen as
doubtful. With the exception of the wonderously preserved Haldonodon from
Portugal, fully paid up docodonts are not well known. Typical fossils are teeth and jaw
fragments from the Upper Jurassic of North America. An increasing range of fossils from
Asia seem to be fleshing out a distinctive family of docodonts called Tegotheriidae.
Woutersiidae and Docodonta
Haramiyidans
The dental details show these critters were predominantly herbivores. The teeth have fine
quantities of cusps on them, reminiscent of the multituberculate mammals. An ancestral
relationship between the two lines has some support. One problem though is the lack of
evidence. Furthermore, what evidence there is tends to say 'probably not'. These haramis
chewed in a side-to-side way. Multis were backwards and forwards chompers, which suggests
their jaws must have been hinged very differently. This could perhaps have been a dramatic
bit of reengineering from the workshop of evolution. If so, there's no fossil evidence for
it yet.
It's not even clear whether these critters were less, more or about as derived as the most
basal mammals. They might be non-mammals according to even the stretchiest of definitions.
Haramiyida
MULTITUBERCULATES
This subdirectory covers the best known of Mesozoic mammals. The name might be difficult
enough to remember, but it appears fairly often in books which include mammals at all. They
were herbivores to omnivores, with lots of cusps on their teeth, thus the name. They also
had a couple of tusk-like lower teeth, and upper incisors reminiscent of rodents. Multis
probably led a similar kind of lifestyle and survived well beyond the Mesozoic. They
finally seem to have been replaced by rodents about 40 million years ago, leaving no
descendants. The first proto-mices turn up during the Paleocene, which is the time
immediately following the extinction of Tyrannosaurus and all. This catastrophe had
no noticeable effects on the multis. They were diverse both before and after it. As
rodents became more common though, multis declined. They didn't go out with a bang, but
more amidst a growing chorus of squeaks and eeks.
The oldest (unnamed) multituberculate fossils are from the Middle Jurassic of England. The
earliest substantial material comes from Guimarota in Portugal, a site which is Upper
Jurassic. A bit later, allodontid multis turn up in the rocks of North America. In the
Mesozoic sites where multituberculates are found, they're generally the most common and
diverse of the mammal fauna. This probably reflects their role in the northern hemisphere
at the time. It also makes northern mammal sites where multis haven't been found intriguing.
However, the explanation might be 'not yet found'.
According to the most recent scientific view, most multituberculates can be reasonably
assigned into two suborders. The most basal members are within 'Plagiaulacida'. The
apostrophies indicate that this doesn't represent a natural group; it doesn't include one
ancestor and all of its descendants. It's the safest interpretation presently possible
with the fossils available. These creatures inhabit the first two directories. The
chronological range of the group extends from the Middle Jurassic until the Lower
Cretaceous.
Basal Multituberculata
Plagiaulacidae, Albionbaataridae, Eobaataridae & Arginbaataridae
One Lower Cretaceous genus from Mongolia, Arginbaartar, shares characteristics with
these plagis. It seems to have its origins within or near to the allodontids. It also has
characteristics in common with the more derived multis, the cimolodontans. These facts do
not mean it must represent a link between the two groups. Cimolodontan genera such as
Paracimexomys are under suspicion.
Cimolodonta seems to be a monophyletic suborder. That means it includes one ancestor and
all of its descendants. Its most basal members are the just cited Paracimexomys and
its kin. This suborder includes all the more derived multituberculates. This means that
cimolodontans survived from the Lower Cretaceous until at least the Eocene, (35 - 55
million years ago).
'Basal' Cimolodonta, Cimolomyidae, Boffiidae and Kogaionidae
Whilst the relationships of multis to each other is quite well known, their placement within
the broader scheme of Mammaldom is unclear. Their descendants are easily described; none,
dead, gone. Not a sausage. The best available candidates as their ancestors are the
haramiyidans, but this seems far from convincing. There are apparently some similarities
in the braincase and the internal ear structure of multis and monotremes, (platypus and Co),
but that's about as far as it goes. For reasons I don't pretend to understand, certain
aspects of skull and limb anatomy suggest these things were more 'primitive' than some
lineages confined to the Mesozoic. Nevertheless, they out-survived them - and why not? We
and our egg-laying monotreme cousins are more closely related with each other, than
either of us are to these multis.
And this points to an interesting sting in the tail; or in the hips. Where known, the
pelvis of multituberculates is rather narrowly built. It seems too narrow to have enabled
the females to lay eggs. This suggests, where such evidence has been preserved, that at
least some multis reproduced a lá live birthing. Given our nearer relationship with
monotremes, this implies to me that live birthing must have evolved at least twice within
mammalian history. Either that, or they produced extraordinarily tiny eggs.
Djadochtatherioidea
Eucosmodontidae, Microcosmodontidae and Taeniolabidoidea
Ptilodontoidea
Multis are more or less restricted to the northern hemisphere, with the exception of some
fossils from the Upper Cretaceous of Patagonia. These were described as ?Ferugliotherium,
which is a genus within Gondwanatheria. This group has been assigned to the multis in the
past, but that's not the present view. They may be related to the predominantly South
American, tooth-poor edentates; anteaters and the like. However, some poorly preserved
material is apparently multituberculate. Although the gondwanatherians don't belong in this
subdirectory, I've left them there anyway, at least until better information is available.
Gondwanatheria
In the case of multis, I utterly ignore the implied restriction of the term Mesozoic, and follow
them to their demise. Given the numbers involved, it seemed pointless not to.
Multituberculata
TRICONODONTS
This is another rather slippery term based on dental characteristics. It's to do with three-
coned molars, which is a typical enough mammalian condition. Some interpretations have
included those early morganucodontids within this group. Parts of Triconodonta seem to be
genuinely related, whilst others aren't.
Some of these animals were giants amongst the mammals of the time. They grew to cat, and
even dog-size. Others were the more usual mouse-small and smaller. Good remains have been
found, including one virtually complete representative, Jeholodens. The main
habitats of triconodonts were in the northern hemisphere of the Upper Jurassic and Lower
Cretaceous. Some North American representatives are known from the lower Upper
Cretaceous.
I've arranged the entries into three sections:
- The first category is a hold-all for triconodonts of some kind or other. It
contains bits and pieces of various groups. Most are 'amphilestids', which is a term based
on some fairly bland characteristics. There is a proposed family, but it's no longer
convincing.
- Some mainly Asiatic 'amphilestids' of the Lower Cretaceous are close relatives of
each other. I've put them into an enclosure for gobiconodontids, (which sounds like
something out of Star Wars), and Repenomamus.
- My third paddock is Triconodontidae. With a couple of possible exceptions, these
animals appear to be members of the same family. The aforementioned Jeholodens is
held to be at least close, if not even closer.
Some of these triconodont fossils help relate a postscript to the 'reptile-mammal'
transition. When the mammalian jaw joint had become the only hinge, the 'reptilian' joint
bones didn't simply disappear. They'd already been involved in the sense of hearing, and
that's the career they pursued. They became the incus and the malleus; small bones inside
the ear. The tiny contemporary of the Chinese morganucodontids, Hadrocodium, is the
earliest known possessor of such sophisticated listening devices. However, this still left
other pieces of reptilian paraphernalia lying around. There was a thing called Meckel's
groove on the inside of the lower jaw. Generally, it contains a cartilage which is involved
in mouth opening. Those small bones form from the end of it. With their new place of
residence in mammalian ears, this link severs.
Although we no longer have Herr Meckel's most excellent groove, his cartilage boogies on.
It formed our original embryonic jaw, before bone replaced it to do the job properly. We
have small bits of it in our ears, but most of it was reabsorbed into the jaw and the
groove never got groovy.
Repenomamus has a cool Meckel's groove. There are even remains of the fossilized
cartilage. It presumably had some use for it. Those closely related Star Wars characters,
the gobiconodontids, also grooved, as did other Jurassic and Lower Cretaceous
triconodontids; eg. Arundelconodon. It's not known from later representatives
though.
This ungrooviness seems to have taken hold in parallel among various mammal lineages,
though at differing rates. Lower Cretaceous representatives of our own line, Eutheria,
were also Meckelian groovers. Sooner or later, we all found it old hat and it faded away.
That's fashion for you.
Most triconodonts, (including the 'amphilestids'), probably fit within Crown-group
Mammalia, somewhere towards the start. Crown-group Mammalia is the most recent common
ancestor of existing mammals, and all of its descendants. Amongst the more tightly defined
Triconodontidae, there are a number of characteristics reminiscent of those egg-laying
monotremes, though other factors don't support a relationship of any intimacy.
Triconodonta
CROWN-GROUP MAMMALIA, THE ROAD TO THERIA
TINODONTIDS AND SPALACOTHERIIDS
These two families aren't closely related, but they were contemporaries. They're also both
'symmetrodonts'. The first mentioned are more 'obtuse angle' critters, whilst the latter
are 'acute angle'-rs. They have reasonably 'advanced' lower jaws, in comparison to the old
fashioned kuehneotheriids. They're also very possibly within Crown-group Mammalia.
The tinodontids were late Jurassic - early Cretaceous residents of North America, Europe
and perhaps Mongolia. The jaw remains show they're more closely related to us than they
were to basal mammals.
The exotic sounding spalocotheriids had much the same geographical and chronological range.
It's clear that they do form a valid family. Fossils are generally limited to odd scraps
of jaw and isolated teeth. However, these have enough peculiarities to show kinship.
There's one superb find from China called Zhangheotherium. It's either a member of
this family or closely related. In some respects, this fabulously preserved fossil's
rather dull.
Up until 1997, all 'symmetrodonts' were based only on jaw fragments and teeth. Trying to
reach any conclusions about the rest of the body was down to educated guesswork. A
reasonable bash might have been: shrewish in size, limbs splayed out somewhat -(in line
with other groups with broadly similar jaws, and unlike types with more 'modern' ones)-,
insectivorous; something like that. When the virtually complete Zhangheotherium was
described, the guesswork was broadly correct. About the only major surprise it offered was
its excellent state of preservation. This is a bit like getting a birthday present, you
desperately wanted thirty years ago. That's unfair. It's a remarkable fossil. It's been
overshadowed somewhat by the even more fantastic finds from its home of Liaoning.
Comparison to a second specimen suggests that the original specimen very possibly represents
a male. This is on account of a spur of bone on the ankle, which the more recently found
fossil doesn't possess. In monotremes at any rate, this is a sex based difference. In
general, this feature is restricted to Mr platypussies and echidnas.
Tinodontidae and Spalacotheriidae
MAMMALS AMONGST THE PALEO-LIZARDS OF AUS
The next two directories have strong Australian connections. The structure is inspired by
a somewhat controversial viewpoint. As these tribosphenic teeth are so unusual, it was
assumed that they could have evolved only once. A relatively recent theory says: hang on.
What if they evolved twice?
A clever bit of interpretation, backed up with an immense amount of observation and testing,
strongly suggests there are significant and characteristic differences between 'northern'
tribosphenic teeth, (Boreosphenida), and 'southern' ones, (Australosphenida). Whilst
broadly similar in function, the construction is not the same. This implies the dual
evolution of tribosphenic molars. Unfortunately, I haven't got any idea whether the Dorset
Tribactonodon is anatomically a 'northerner', a 'southerner' or somewhere in between.
Based on common sense, I expect it'll be the former, but this is a question of anatomy. In
my present ignorance, I dumped it here.
The inclusion of Shuotherium is also controversial. All lower tribosphenic molars
have a grinding surface known as a talonid. Those of Shuotherium have no such thing.
Instead, there's a pseudo-talonid. It's a similar sort of structure but in an unexpected
position. This is situated in front of a feature known as the trigonid, which is the
reverse of the tribosphenic pattern. It 'should' be behind. I'm following the notion that
this Chinese (and British) creature represents a sister line of these southern
australosphenidans. However, I'm increasingly impressed by another line of thinking, which
links shuos with docodonts.
The third grouping on the first directory of Aus contains the down under connections, and
the attractions of the dual origin theory. This section is home to the earliest known
australosphenidans and presently houses four genera. Two come from the Middle Jurassic of
Argentina and Madagascar. The other couple are from the Lower Cretaceous of Australia.
This pair is also a matter of debate. An alternative view is that they're early placental
mammals. This is partly based on the dental formula and some similarities of the jaw
construction with hedgehogs. I'm presently more heavily influenced by the dual originist
view. This interprets Australosphenida as including
Monotremata; the duck-billed platypus and echidnas. However, subsequent research
suggests that monotremes don't, and never did have tribosphenic
molars. If correct, than they can't be australosphenids.
One reason why there's so little consensus on these issues is that there are so few fossils.
Until the last few years, very little Mesozoic mammal material had been recovered from the
southern hemisphere. This is beginning to change.
Tribactonodon, Shuotheriidae and Australosphenida
MESOZOIC MAMMALS? THE PLATYPUS AND FRIENDS
I had a lot of fun compiling this next directory. It starts off amongst the usual scraps
and teeth of the Mesozoic. The fossil record of long dead relatives of the platypus isn't
extensive, but it's slowly improving. One unexpected novelty is provided by a couple of
slightly post Mesozoic teeth. Monotremes are confined to Australasia. There was no record
of their ever having had a broader range. This changed in 1992 with the publication of
Monotrematum, which turned up in Patagonia. Increasing numbers of paleontologists
enjoy working there.
Whilst this find was surprising, the presence of platy-ancestors in South America is in
line with the geography of the time. It was linked to Australia via the Antarctic. That's
the route marsupials used to reach Aus. Although 98% covered by ice, Antarctica boasts a
number of extinct marsupials. It's even yielded bits of an Eocene sloth. Apart from being
larger, these Patagonian teeth are similar to those of the later Australian, Obdurodon,
which was clearly a close relative of the modern platypus. The quantity of web material
available on the platy is predictably enormous.
There are a couple of other egg-laying mammals, though they're not as famous. These are the
echidnas, which look something like large hedgehogs with a touch of anteater. One of the
first Westerners to meet an echidna was Captain Bligh, shortly after his enforced departure
from HMS Bounty. He made a sketch before dinner. Echidnas are very easy to catch, tasty
and have an entertaining sex life.
As with the multituberculates, I strayed way beyond the Mesozoic with this group.
Monotremata
DRYOLESTIDS AND COLLEAGUES
These creatures started out as a northern group of insect crunchers. There are also several
African representatives from the Upper Jurassic (Tanzania) and the Lower Cretaceous
(Morocco). However, the associated floral and faunal fossils are also broadly 'northern'.
Probably the most peculiar feature of dryolestids was their enthusiasm for enormous numbers
of teeth. Unless you've lost any, you've got two molars on either side of your upper and
lower jaws. The wisdom tooth represents a third, and that's as many as any placental.
Marsupials luxuriate with up to four. Dryolestids wouldn't have dreamt of leaving home with
less than seven, and a few even went as far as nine. That's on each side.
Collectively, dryolestids, paurodontids and the like, (plus critters such as the
prototribosphenidans, below), are also known as eupantotheres. Knowledge is usually
restricted to jaws and teeth, which are generally Upper Jurassic - Lower Cretaceous. A
spectacular exception is the near complete paurodontid, Henkelotherium, which is
from Guimarota in Portugal. (Paurodontids were somewhat less enthusiastic in their taste
for molars, though they were also pretty toothy.) The skeleton suggests a life in the
trees. The construction of its legs is more 'advanced' than is the case for the living
monotremes.
Freaky dryolestids took part in the 'Jurassic Revival Festival' of Upper Cretaceous
Patagonia. Some of these ravers made it into the Paleocene. Quite where these lines were
hanging out for much of the Cretaceous isn't known. Suitable southern hemisphere fossil
sites are doubtless out there somewhere, so please go and find them.
Dryolestida
PROTOTRIBOSPHENIDANS AND THE LIKE
Prototribosphenic mammals were those with nearly tribosphenic molars. That's those grinding-
cutting teeth mentioned above. The ancestors of the northern tribosphenidans would have
been amongst them, and they probably looked quite like a genus called Peramus, from
the Lower Cretaceous of Dorset, England. It's amongst the most basal looking members of a
grouping called Zatheria, which begins with the most recent common ancestor of
Peramus and myself. Of course, the ancestor would have lived earlier, and
stem-zatherians have now been identified from Upper Jurassic Guimarota, Portugal.
As well as dental developments, there was some significant change underway in the inner ear,
as revealed by the Lower Cretaceous South American, Vincelestes. Back at
Morganucodon, I mentioned an elongated cochlear canal. That's the thing sound waves
travel along. In ourselves and marsupials, this canal is significantly longer. It's
coiled. In the case of Vincelestes, this already exceeded 270°, which is a more
pronounced coiling than is known from those egg-laying monotremes. They've also further
enhanced their hearing abilities, but they used a different trick. Rather than lengthening
the bony canal still more, monotremes have worked on their aural membranes. Amongst other
things, this might suggest that these prototribosphenidans were effectively
proto-northern-tribosphenidans. The southerners had branched off long before.
Stem-zatherians, zatherians and Peramuridae
BOREOSPHENIDANS AND THE EMERGENCE OF THERIA
This is where the northerners finally catch up with their dentally sophisticated cousins.
The southerners had already invested in high tech, dual purpose molars back in the Middle
Jurassic. Our ancestors didn't get with it until the Lower Cretaceous, (or perhaps
somewhat earlier). Maybe they'd been too busy working on their bony cochlear canals.
This is also an area of intermingling shades of grey. People like tidy stories; beginning-
middle-end. With that in mind, from the boreosphenidans emerged the therians, ('beasts').
The therians branched. One line carried on for a while as therians. A second evolved into
eutherians, (placentals plus a bit). A third led to metatherians, (marsupials plus a bit.
Did I mention marsupials were originally northerners?).
The boreosphenidans are known from the lowest of the Lower Cretaceous. As is to be
expected, the therians, basal eutherians and basal metatherians were all broadly similar.
There are clear diagnostic differences between placentals and marsupials. However, these
distinguishing characteristics crystallized over many generations. This can make
recognizing early representatives extremely difficult, especially when most of the material
available is fragmentary.
Boreosphenida
Theria
Repetition for the sake of clarity. Basal therians + the eutherians (= placentals plus) +
the metatherians (= marsupials plus) = Theria.
MESOZOIC MARSUPIALS
Until December 2003, the known fossil record contained a eutherian which was 25 million
years or so older than the earliest known metatherian. There's just been an equalizer. A
beautiful specimen from Liaoning, China called Sinodelphys has dramatically filled
a gap in the ancestory of marsupials. It's 125 million years old and also now shares the
record for the earliest direct evidence of fossilized fur. If anyone would care to
uncover an earlier, undoubted therian this would be most welcome.
The cast list of Mesozoic metatherians is composed of opossums and proto-possums. Cuddly
kangaroos and sex-mad, male koalas arose much later. It also begins in their nurseries of
Asia and North America. The only evidence from the southern hemisphere is an Upper
Cretaceous tooth from Madagascar. (Update: I'm just printing out a new paper. This tooth
may not be from a marsupial.) I've heard of South American sites, but they generally
turn out to be slightly post-Mesozoic. However, given the diversity of Paleocene
marsupials of that continent, there's a reasonable likelihood that some Cretaceous
representatives were endemic.
The first directory provides sanctuary for 'basal' metatherians, deltatheroidans and
asiadelphians. the 'basal' members the aforementioned Sinodelphys and the earliest North
American representative, Kokopellia. This genus is based on some bits of 100 million
year old jaw from Utah and Oklahoma. They're accompanied by isolated Upper Cretaceous,
North American teeth.
Deltatheridium is a reasonably well preserved creature from the Upper Cretaceous of
Mongolia. It has some marsupial characteristics; eg. part of the dental formula. However,
it lacks other marsupial details. As a consequence, it's been interpreted as both a basal
boreosphenidan, (albeit a late one), and a basal metatherian. However, recent research has
confirmed its placement within Metatheria.
The best known asiadelphian is Asiatherium from the Upper Cretaceous of Mongolia.
Unfortunately, I don't yet have much information available on these doubtlessly charming
critters.
'Basal' metatherians, Deltatheroida and Asiadelphia
The second instalment concerns mainly North American metatherians of the Upper Cretaceous.
The first group's called Stagodontidae. They seem to have been brought to an end by the
K-T extinction events which accounted for the last dinosaurs, (non-birdy). One of their
number, Didelphodon, is among the largest known of Mesozoic mammals. It was roughly
badger-sized, and managed to negotiate a cameo role as an egg eater in the last episode of
Walking with Dinosaurs.
They're partnered by their friends and relatives, the pediomyids. These are also unknown
in the north beyond the Cretaceous. However, as one of the family, Monodelphopsis,
has turned up in the Paleocene of Brazil, they seem to have been involved in spreading the
marsupial message to the south.
Stagodontidae and Pediomyidae
The final Metatheria directory features 'basal' Didelphidae. These are full-blown opossums
and their origins also seem to be in the Upper Cretaceous of North America. They became
very cosmopolitan. There's little to indicate that they fared any better than the
stagodontids in the north. However, like the pediomyids, they also spread south. Later,
the Virginia possum returned to its roots. It's also in this family.
As is Peradectes. I'd be surprised if all the reports of this genus are accurate.
It's perhaps known from the Cretaceous of both Americas, (though I have my suspicions
concerning the correctness of this). It has most definitely been recovered from the Eocene
of Europe and possibly Africa. Peradectes is one of several marsupials from the
fine fossil sites of Germany, (eg. Messel near Darmstadt).
In view of the diversity of post-Mesozoic Marsupialdom, I felt compelled to more or less
stop at the Mesozoic, which is in many ways a pity.
'Basal' Didelphidae & Co
MY ANCESTORS
THE DAWN MOTHERS
Eutheria contains all placental mammals and our immediate ancestors. As with Metatheria,
the oldest and most basal known representative kicks this group off with a bang. It's
another absolute stunner from Liaoning, China.
Eomaia was described in 2002. Its name means 'dawn mother' and it's about 125
million years old. If anything earlier ever turns up, I suppose it'll have to be called
'night-before granny' or some such. As it's exquisitely preserved, it also substantiates
a number of details concerning early eutherids.
It was reasonable to assume that all mammals must have had at least some hair, even though
such a feature almost never fossilizes. 'Almost never' is a synonym for 'occasionally
does'. The oldest direct evidence was provided by a 60 million year old, Chinese
multituberculate named Lambdopsalis. Eomaia doubled that figure. Extensive
traces of fur and finer hairs are well preserved.
Where evidence exists and with the general exception of placentals, all mammals have or had
epipubic bones. These stick out from the pelvis and are also known as marsupial bones.
Mrs Kangaroos use them as pouch supports. They probably started out as leg muscle anchors
in pre-mammalian days. (They're known from those strangely named tritylodontids. Most
mammals today have no such things, but that's because they're placentals. Eomaia
had them, as did various basal colleagues from Mongolia, (Lower and Upper Cretaceous).
Another detail, which had already been observed in several of these Mongolians, concerns
the teeth. Those between the canine and the large cheek teeth are called premolars. The
maximum count for all existing placentals is four per side. As some basal, but clearly
eutherian Mongolian fossils had five, it was reasonable to assume, that the earliest
representatives would have had at least that number. Eomaia has five.
The pelvis of Eomaia is relatively narrow. This suggests live birthing, but with
poorly developed babies, a pattern known from marsupials. As the babies would have been
small, they wouldn't have enjoyed a prolonged residence in the womb, which means they'd
have had no use for a placenta. This suggests that Eomaia, though certainly a
eutherian, was a kind of pre-placental placental mammal. As this method of reproduction
was followed, or is generally followed, by Eomaia and marsupials, it's reasonable to
say this must have been the original, ancestral pattern; the therian method. Placental
assisted birthing developed later, (as well as independently and differently amongst a few
marsupials).
As far as Herr Meckel might be concerned, Eomaia was certainly a groover.
This is not only one of the most astonishingly beautiful fossils ever found. It's also very
obliging in the way it conforms to and confirms many expectations. That's why I've given
it these few hundred words.
More may be enjoyed or jeered at on the first Eutheria directory. It's a collection of
basal representatives, hard-to-placers and odd bits and pieces. It also contains some info
on the earliest North American eutherian, Montanalestes, and a photo of a
particularly cute European representative, (Self MY).
'Basal' Eutheria
THEIR CHILDREN
As far as I understand the concept, (which isn't necessarily any further than the distance
to the nearest door), Epitheria is something akin to all eutherian mammals except for the
most basal ones, and the line which led to the predominantly South American near-toothless
wonders of Xenarthra, (the aforementioned edentates). However, I've decided to abandon
attempting to use the category and have adopted a looser approach. All the following are
reasonably described as small insectivores.
The first section is a basket of critters which might belong elsewhere. As is so often the
case, interpretations vary. It's a small Asiatic collection, ranging from the Lower
Cretaceous to the Upper. It's the sort of section I enjoy trying to close down, as and
when possible. The second group is Asioryctidae, which comprises a couple of reasonably
well-known genera. They're possibly related to the zalambdalestids on the final directory.
Finally, we have the gypsonictopids; an exciting sounding group. Prokennalestes is
another Lower Cretaceous eutherian, whilst its companions are from the Upper Cretaceous of Asia and
North America. The structure of the page is fairly described as liable to subsidence.
Basal Eutheria Two
OF HOOFS AND DOLPHINS, ELEPHANTS AND FINS
There are differing views on the scheme I'm following for this next collection, but it
provides a very convenient structure, a fine narrative context and some great links. It's
one interpretation and reasonable alternatives are certainly possible.
Ungulates are more generally understood to be animals with hoofs. There are odd-toed
ungulates, (eg. horses), and even-toed representatives, (eg. sheep). Ungulatomorpha unites
all imaginable ungulates together with some more surprising kith and kin; elephants, sea
cows, aardvarks and whales count amongst the latter. The thinking is that all these animals
have a common ancestor, and possible candidates have been dug up in the Upper Cretaceous
rocks of Central Asia, Europe and North America. Whilst there's enough evidence to support
a 'maybe', I wouldn't wish to put it more strongly than that. This centres on things often
called zhelestids, which may, at least in part, be closely related with the zalambdalestids,
(below).
Perhaps surprisingly, the controversy doesn't involve the connections between whales and
ungulates. That's more a matter of which ungulates. The extinct, carnivorous and
terrestrial mesonychids are favourites, whilst proto-hippos also have some support. There's
also a further candidate for the mum-gulate; Protungulatum of the Upper Cretaceous-
Paleocene of North America. It's the placement of the slightly earlier fossils that's less
clear. They're perhaps best described as 'could be-s' in terms of proto-hoofers, but they
might represent extinct lineages. Or something else.
'Basal' Ungulatomorpha
PROTO-MICE AND PROTO-CATS?
The inhabitants of this final directory are also more tidly arranged than they might deserve.
I've followed a couple of possible scenarios.
There is some evidence that zalambdalestids, (further furry inhabitants of the Central
Asiatic Upper Cretaceous), could be the forerunners of Glires, which is composed of rodents
and bunny rabbits. All members of Glires share a suite of dental characteristics, some of
which are also evident in the thrilling sounding zalambdalestids. This could be a matter of
ancestry. I've gone along with this 'could', but don't intend it as meaning 'is'. They
look like perhaps ancestors.
I've followed a similar line with cimolestids, though I doubt that many of the included
'members' ever paid their joining fees for this family. The version of Cimolestidae I've
adopted looks suspicious. Nevertheless, Cimolestes has been accused of being the
inventor of all cats; and even of all carnivores. Given that the forerunners of all
existing mammals were small, squeaking insect-eaters, it might be guilty. If not, something
broadly similar was.
Zalambdalestidae and Cimolestidae
FINAL CAUTION
As this is intended as a rough and not necessarily reliable sketch, I'm not including any
references, but am indebted to many sources. These are listed in the bibliographies of the
individual directories.
My reasoning is as follows: people with less knowledge of this subject than I have, might
gain the impression that I'm some kind of expert. Those with more knowledge can assure
them that I'm no such thing. My knowledge is superficial. If I listed references, it might
make this essay look like some kind of academic endeavour. It's not. It's a hobby piece
for my own interest. Mind you, I take hobbying seriously. This overview may also
sometimes slip out of kilter with the directories, which are being constantly updated.
Trevor Dykes (not a paleontologist)
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