|MESOZOIC MAMMALS; 'basal' Eutheria, an internet directory:|
PLEASE NOTE: THIS PROJECT IS NOT SCIENTIFIC. IT IS A HOBBY.
"I was looking for information on an old mammal and found this lot. What is this project?"
It's got lots of information on old mammals. For a short bit of background information, see here.
Looking for books?
You could visit the Book Centre and look around.
Congratulations, it's a placental. (Actually, that's
not quite accurate. All placentals are eutherians, but Eutheria is wider than
If we accept the morganucodontids as being the first mammals, then it seems to have taken about 90 million years for Eutheria to emerge, though an earlier birthdate shouldn't be ruled out; Eomaia, (May 2002), Barremian, ca. 125Ma. Some of these fossils represent the presence of the major branch of we modern mammals. Still small though they were, their descendants would come to dominate the world, diversifying to take advantage of an astounding variety of ecological niches and inventing football and cricket. The most practical way of constructing this page for now is as a straightforward a-z.
Mikko Haaramo's Eutheria
A thoroughly good piece of thoroughness.
A somewhat later and more derived eutherian.
T Mike Keesey, The Ages of the Mesozoic
The chronology of the Mesozoic made clear.
|Taxon: Eutheria Huxley, 1880|
Remarks: "Eutheria Gill, 1872 covered both marsupials and placentals and until recently lay fallow. Eutheria Huxley, 1880 covered "placentals" and putative "placentals", but not members of the Marsupialia as well," (McKenna & Bell, 1997).
Eutheria literally means 'true beasts'. I'm not sure that
other beasts would appreciate the arrogance of this assertion, but I don't suppose they
mind. They can't read.
Platform "Iepen Blijven"
A photo album of various modern eutherians.
"Beleutinus" (nomen dubium),
Kennalestes (partly = Daulestes), Kumlestes (= Daulestes),
Majoria (= Plesiorycteropus), Montanalestes,
Myoryctes (= Plesiorycteropus),
Prodelttheridium (nomen nudum),
Eocene: Eodesmatodon, Eutrochodon, Helioseus
Paleocene: Obtususdon, Tingamarra, Wanotherium
Upper Cretaceous: "Beleutinus", Cretasorex?, Telacodon, Uzbekistan
Lower Cretaceous: Acristatherium, Eomaia, Montanalestes, Prodelttheridium (nomen nudum)
Upper Jurassic: Juramaia
Hu Y, Meng J, Li C & Wang Y, 2010|
Remarks: The name refers to the lack of a sagittal crest on the skull.
|Species:||Acristatherium yanensis Hu et al, 2010|
|Place:||Yixian Formation, Liaoning|
|Age:||lower Aptian, Lower Cretaceous|
|Remarks:||The following is based upon my reading of Hu et
al, 2010. My copy, happens to be dated as 2009. That was the date of the
on-line publication rather than the hard copy. Being British, the Royal Society takes
delight in publishing papers the year after their first appearance, and did so prior to
the advent of the internet. This paper will doubtlessly be cited as dating from 2009 but,
strictly speaking, that would be wrong.|
The lead author, Yaoming Hu, is reported as having died on 12.4.2008.
A welcome for the grandmother of the dawn
One of the treasures to have emerged from Liaoning's Yixian Formation is an outrageously complete skeleton of a Lower Cretaceous mammal named Eomaia. When that was published, checking reminds me it was 2002, it was the earliest and most basal eutherian ever seen. A human interest side to that story is that humans, unlike marsupials, are also eutherian mammals. Eomaia was close to the cradle from which, generations later, placentals, crawled out from. A new skull from the Yixian Formation, Acristatherium, has added more notes to the melody. Although similar in age to Eomaia ('dawn mother'), Acrista is somewhat more basal within Eutheria; a representative of the dawn grannies.
This new eutherian comes from Yixian's Lujiatun beds, a different locality than Eomaia's, but one of much the same age (p.229). By Yixian standards, merely a skull could be termed miserly. However, that's in the face of the Formation's wild extravagance with providing entire corpses. Another contrast is the 3-D state of preservation in this case. Upper Yixian mammals have a tendency towards flat-as-a-pancake-ness. This happenstance allows for the revelation of some new details requiring three dimensional preservation.
With a skull length of 2.5cm (p.230), the size of the critter compares neatly with a mouse. An oddity is a bit of narrow bone between the premaxilla and nasal. This doesn't appear to be a branch from either the maxilla or nasal, and happens to be in the right sort of position for a vestigial septomaxilla. Eutherian skulls don't typically have such things. Confirmation of its presence and identity from other early eutherians isn't presently available.
The area at the top of the frontal bones is described as bulbous, and that ought to reflect the storage requirements for the former content; plentiful space for large olfactory lobes of the brain. That suggests a strongly developed sense of smell.
The dental formula per side is (p.231):
Uppers: 4 incisors, 1 canine, 5 premolars and 3 molars.
Lowers: 3, 1, 5 and 3 respectively.
This formula is classically basal eutherian. Later ancestors of we placentals did away with one of the premolars. Also notable is that, while the molars exhibit some wear, the teeth in front of them don't. Therefore, they're more recently erupted and are the second generation teeth; the adult set. The lack of wear pins this individual down as having died as a young adult. If they had a sex life to enjoy, then it can only have involved early escapades.
Incisors and canines
All incisors are small, simple and single-rooted teeth with the lowers being procumbent. That means they slant towards the front. The middle incisors, namely I2-3 and i2, are a bit larger than their neighbours, but not by much. The fourth upper is mainly housed in the maxilla with perhaps some contact with the premaxilla at the front. We modern mammals keep all such teeth within the premaxilla or, in some more senior cases, overnight in a glass with suitable cleansing fluid. As far as I'm aware, that's a dental derivation restricted to humans. Lower incisors are a bit bigger than their upper counterparts.
Canines are much taller than their neighbouring teeth and, like the incisors, only have one root. The upper, in particular, looks delightfully vicious. It gets termed "dagger-like"; an elegantly curving blade culminating in a sharp point and, consequently, weaponsmiths might object to the comparison. While it has no blood-sucking pretensions whatsoever, I'm moved to muse that Christopher Lee would've loved a larger version for biting the necks of large-breasted actresses whilst portraying Count Dracula in the movies or, who knows?, in the privacy of his own personal castle. The prop department responsible for equipping Acrista was clearly more on the ball than their Hammer Horror colleagues.
Both upper and lower premolars are relatively lightly packed along the jaw; ie. there are spaces between them. As the jaws have been preserved in tight occlusion, and separating them would inevitably result in damage to the only known specimen, the reason for that spacing is clear to see. P1 fits between p1 and p2, P2 fits between p2 and p3, and so on. That spacing allows for the reception of the corresponding set of premolars. All these teeth are double-rooted.
Most premolars are single cusped teeth, although P4 and 5 (uppers) have additional cusples at the front and rear, as does p5 (lower). The p4 also has a rear cuspule (p.232). The simplicity of the construction debars terming the latter premolars as even semimolariform.
The count of three molars per side, both upstairs and down, is also classical for basal eutherians and, indeed, still a popular fashion for placentals. It contrasts with the basal metatherian complement of four.
The best preserved of the uppers are those in the first two positions. Of these, M2 is a bit larger with both teeth being a touch wider than long. A wide stylar shelf graces the external side of the crown with an ectoflexus bay of modest depth on M1; a bit deeper on M2. The front lobe of the embayed stylar shelf is larger than the rear one. The stylocone has been worn down low while the parastyle cone is large. The metacone is small and a further tiny cone occurs prior to the parastyle. The protocone, which occludes with the talonid of a lower molar, is a small cusp. Only part of the third upper molar has survived the years. All of these teeth seem to be triple-rooted.
The smallest lower molar, m1, happens to be the best preserved. The second isn't much larger. trigonids are higher, more lengthy and broader than the talonids. The protoconid wins the height contest for the trigonid cusps, while the paraconid is lower than the metaconid. A large cusp e also features. The talonid has a distinct hypoconid and entoconid with, about midway between them, a small hypoconulid.
An old head on what were young shoulders
The construction of the roof of the skull, discussed far more in the paper than my very lightweight notes might seem to imply if misconstrued, is similar to that seen in other early eutherians; eg. long nasals, large frontals and parietals that are bulbous. As much the same can be said for metatherians and lots of non-therian Mesozoic mammals (including, if termed mammals, ancient ancients such as Morganucodon and Sinoconodon), this may sensibly be termed eutherian retention of a style inherited from the most ancien regime. To me, a puzzling feature is the vestigial septomaxilla on the upper jaw, should that interpretation indeed be correct. Such a thing hasn't previously been found on an ancient therian. (A septomaxilla has been recorded for dasypoid marsupials. However, that appears to be a new feature in a similar position rather than a retained character.) The septomaxilla has been reported for Sinoconodon, Gobiconodon, Repenomamus, Vincelestes, monotremes and, disputedly, by Hahn for a paulchoffatiid multituberculate. In that latter instance, not mentioned by Hu et al, another interpretation is an artefact resulting from damage.
In the case of dawn granny, should it be a retention, the bone seems to be derived from just the rear of the ancestral septomaxilla, and a similar structure ought to have been present on other basal eutherians and, indeed, ancient therians. The fact that confirmation for this is lacking could simply reflect a chronic shortage of suitably preserved remnants of the appropriate area of the upper jaw. Another possibility, not discounted by the authors, is that their identification of the feature is incorrect.
Several tooth points
The number of incisors, four up and three down, has been done before by a Cretaceous eutherian; Kennalestes. More common among known genera is a toothier five-four. Eomaia provides a Yixian exponent of that. Presumably, the latter case is the ancestral condition for eutherians. Large, single-rooted canines are widespread among both basal eutherians and metatherians, and that's the fashion Acrista follows (p.233).
When it was discovered that early eutherians had five premolars per jaw half, it came as a surprise. Four was the anticipated original count. One suggested explanation for the unexpected quantity, was that the third tooth could've been a retained milk premolar. However, deciduous premolars are typically more complex than their replacements. In this instance, the third premolars fit in well as members of a gradated set in terms of function, and nothing suggests they're of an earlier generation than the other premolars.
There is a premolar character that's unusual for basal eutherians. The final lower is often semimolariform; eg. Montanalestes and Bobolestes. While that doesn't apply for Prokennalestes, the upper final premolar is semimolariform for that genus. Neither final premolar shows such habits for Acrista. There's strong differentiation in build between premolars and molars both upstairs and down, a situation that's more usually found with Cretaceous metatherians. However, the aforementioned cuspules at the front and rear hint at a possible tendency towards a more molariform future.
An analysis based upon 70 mammals and 408 characters was run, with 178 of the characters being applicable for the new genus. This indicated that Acristatherium is the most basal known member of Eutheria.
The type fossil, IVPP V15004, is much of a skull entertaining its fans at the Institute of Vertebrate Paleontolgy and Anthropaleontology, Bejing. The specific name celebrates Yanzigou, Acrista's grave locality.
|Reference:||Hu et al (2010), New basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China, Proceedings of the Royal Society B, 277, p.229-236.|
|Genus: "Beleutinus" Bazhanov, 1972|
|Species:||"Beleutinus orlovi" Bazhanov VS, 1972|
|Place:||Bostobe Formation, Aral Sea region|
|Age:||Santonian, Upper Cretaceous|
Averianov & Archibald 2003, (p.14-15), cite this as nomen
dubium, (dubious). Remains consist of a fragment of
dentary with some broken teeth.|
This might actually be a deltatheroidan, which were proto-marsupials. It's also seen as a possible zalambdalestid, (eg Averianov & Archibald 2003). It was originally referred to Zalambdalestidae, but that seems unjustified, (Wible et al 2004, p.16)
The citation details have been taken from the references in Kordikova et al, 2001.
|Reference:||Bazhanov (1972), Pervaya nakhodka mezozoyskogo mlekopitayushchego Beleutinus orlovi v sovetskom Soyuze. [The first Mesozoic mammal (Beleutinus orlovi) from the Soviet Union], Theriologiya, 1, p.74-80, [in Russian].|
Cretasorex Nessov LA & Gureev AA, 1981|
|Species:||Cretasorex arkhangelskyi Nessov LA & Gureev AA, 1981|
|Age:||Coniacian, Upper Cretaceous|
"Nessov (Nesov) and Gureev (Gureyev) believed this almost indeterminate animal to a
Cretaceous shrew because of the pocketed coronoid process of the mandible. However, such a
structure occurs in some mammals that lack a zygomatic arch, e.g. in certain
apternodontids. Cretasorex is apparently not referable to either Sorex or to
Diplomesodon, both of which are currently resident in the Kyzyl Kum area where
Cretasorex was found. That the animal actually represents a shrew remains to be
proven," (McKenna & Bell, 1997).|
An enquiry arrived in December 2002, regarding the possibilities of a three foot high, bipedal insectivore for a fantasy role-playing game. Cretasorex didn't seem a suitable model! The last I heard, Leptictidium Tobien, 1962, (from the Eocene of Germany), seemed more promising.
|Reference:||Nessov & Gureev (1981), Nakhodka chelyusti drevneyshey zemleroyki v verkhnem melu pustyni Kyzylkum. Dokl Akad Nauk SSSR 257, p.1002-1004.|
|Genus: Eodesmatodon Zheng J & Chi H, 1978|
|Species:||Eodesmatodon spanios Zheng J & Chi H, 1978|
|Remarks:||I don't know what this is or why it's here.|
Eomaia Ji Q, Luo ZX, Yuan CX, Wible JR, Zhang JP & Georg JA, 2002|
"First of all, Eomaia is not a placental mammal. It is an early, primitive
representative of the lineage that eventually led to placental mammals," said Anne
Weil, (in the linked article). She also observed: "Eomaia - 'Dawn Mother' - is
exceptionally well-preserved for a 125-million-year-old. Although the fossil's skull is
squashed flat, its teeth, tiny foot bones, cartilages and even its fur are visible."
|Species:||Eomaia scansoria Ji Q, Luo ZX, Yuan CX, Wible JR, Zhang JP & Georg JA, 2002|
|Place:||Yixian Formation, Liaoning|
|Age:||Barremian, Lower Cretaceous|
|Remarks:||The following is based upon
my reading of Ji et al, 2002.|
Until 1999, it seemed reasonably secure to assume the centre of initial eutherian evolution had been in Asia, which provided the only universally accepted, relevant Lower Cretaceous fossils. They were about 100 million years old. Then North America pushed the record back by ten million years with Montanalestes. Asia retaliated shortly afterwards with Murtoilestes from Transbaikalia, Russia. That set the record back to about 120 million years. Then the Yixian Formation of northeastern China yielded 'dawn mother'; a fossil which merits any and all of the following superlatives: stunning, spectacular, staggering, mind-blowing, beautiful, magnificent, unbelievable and so on and so forth. This is a sensationally well-preserved 125 million year old animal, and the skull and skeleton are known almost completely, (p.816).
Prior to this megastar of vertebrate paleontology, the oldest eutherians providing anything like this kind of detail were forty or fifty million years younger. However, Eomaia shows still more audacity. The fur and even some soft body tissue, (eg cartilages from the spine), have been preserved. Apart from at Liaoning, the oldest direct evidence of mammalian fur comes from post-Mesozoic carnivore droppings containing undigested bits of Lambdopsalis.
Although it might sound premature to voice such an opinion, this is one of the most spectacular finds described in the entire twenty-first century. Should that statement turn out to be an exaggeration, I will happily apologise, (if possible). However, I doubt that will be necessary. This fossil merits excitement.
The living world contains three natural categories of mammals. About 270 species, (6%), are marsupials. The three species of egg-laying monotremes are too few to merit a percentage point. The other group are reasonably close relatives of marsupials, and provide around 4,230 species. I'm a member myself. These placentals are a major part of a wider grouping called Eutheria, which contains all critters more closely related to me than with a koala or a kangaroo. While not strictly being a placental mammal, Eomaia is undoubtedly a eutherian. As it's presently both the oldest and most basal representative known, the authors named the genus 'dawn mother'. The specific name, scansoria, refers to scansorial abilities, which means climbing. Features of the legs and feet are only found in clambering and tree-dwelling mammals. This indicates a similar lifestyle for this critter.
How big was it, then?
What matters is quality, not size. These are my estimates based upon the sketches in the description, (p.817 and 818). The skull, (which is partly represented by impressions rather than solely fossilized bone), has a length of 2.5cm; the neck and body contribute 6-7cm (depending upon how curved the spine was in life); and a reasonably straight tail adds around 7.5cm. Precision concerning the length of a tail is startling. There are 25 separate vertebrae involved, and all of them are half a centimetre or less in size. Not one is missing. Just think for a moment about a fragile structure like that and a 125 million years. This preservation of details is breathtaking.
The maximum length of the lower jaw is about 2.2cm. (What's also worth bearing in mind is the care and effort which goes into sketches like these. My estimates are crude in comparison.)
More detailed stuff
Eomaia differs from other Lower Cretaceous eutherians in details of dentary and dentition. Unlike Montanalestes, the lower jaw retains a Meckelian groove. A further difference from most (but not all) eutherians, is that the angular process towards the back of the jaw is angled slightly inwards.
The dental formula is typical for basal eutherians, and unknown from any metatherians; (uppers): five incisors, one canine, five premolars, three molars; (lowers): four, one, five and three respectively. (Later during eutherian evolution, some of those teeth were dispensed with.) In contrast to less derived boreosphenidans, cusps on the talonid called the entoconid and hypoconid are of near equal sizes, whereas the first cited is relatively smaller in the stem animals. As the molars are fully tribosphenic, it obviously differs from non-tribosphenic mammals. The jaw lacks the primitive postdentary trough, and this is a contrast to the later Australian ausktribosphenids, ( Austribosphenos and Bishops).
The wrist contains an enlarged and long trapezium. The hamate bone is large, but not as oversized as in marsupials (and Sinodelphys which was subsequently described). In contrast, the tapezoid and capitate are relatively small, and the proportions of all these bones is similar to the situation found in living climbing mammals. These wrists hosted hands made for grasping, and that indicates climbing.
I've spent much effort studying the hips of a mammal, but scientific enquiry wasn't the objective. Nevertheless, this area of Eomaia contains much of interest, albeit for reasons distinct from my own explorations. "The ilium, ischium and pubis are fused", (p.818). An interesting feature is the presence of epipubic bones. These are universal among mammals excepting for derived eutherians called placentals. As non-mammalian tritylodontids had them too, the characteristic even pre-dates Mammalia, (Reilly & White 2003, p.400.) The build of the hips suggests there wasn't enough room for a wide birth canal, and that points to a mode of reproduction involving poorly developed, new born babies. Not only is Eomaia too basal to be regarded as a member of Placentalia. Embryonic development presumably didn't involve placental feeding.
"The ankle of Eomaia bears strong resemblance to those of Late Cretaceous eutherians..." (reference numbers omitted). It contains a characteristic which is regarded as diagnostic for eutherians. As that concerns details I've no intention of trying to address, I'll just say astragalotibial facet. (Ankle terminology is terrifying.)
Impressions of hair are preserved all round the body. There's even a thin layer on the tail. The densest concentration is nearest to the body, and provided insulation. The outer coat was also equipped with longer guard hairs. None of these details are surprising. The flabbergasting aspect is the fossilization of any hairs at all. At the time of publication, this was entirely unprecedented for Mesozoic mammals. Subsequently, several further furry critters have emerged from the former lakelands of the Yixian Formation. Nobody assumed pre-Paleocene mammals had been nudists, but finding fully dressed specimens is highly improbable. Hair virtually never-ever fossilizes. However, Liaoning specialises in improbabilities.
Fingers and toes
For the purposes of orientation, mammalian hands traditionally contain five (or less) elements technically termed fingers. Each of those has three bones, or two in the thumb. In every digit, the bone nearest to the wrist is a metacarpal, and the other bones are phalanges. The thumb is digit 1 and your little finger is digit 5. You'll have to work out the rest of the sequence for yourself.
Conveniently, other than for one exception, these terms and technicalities also apply to feet, which is why I've taken a bit of time to outline them. (The exception is that toes contain metatarsals rather than metacarpals.) These features help to explain: a. how I can count to 21; b. that I'm male. As the situation is broadly similar in the genus of Eomaia, you can use your own paws as visual aids and, I promise, we'll have no need to count even as far as 20.
The proportions of the phalanges in front and back feet are similar to those known from existing arboreal animals. For example, the claw bearing phalanx of the third finger is relatively long when compared to the other phalanx, (79%). That ratio tends to be lower in terrestrial mammals, and can be higher in full-time tree dwellers.
In the feet, the outer toes (4 and 5) are relatively long. The phalanges, (but not the metatarsals), are longer than the corresponding bones in digit 3. In ground dwellers and clamberers, the phalanges of digits 2 and 3 tend to be the longer.
Eomaia claws are more similar to those of scansorial mammals, (climbers), than with fully arboreal ones. In profile, they compare with the dormouse, (Glis), which favours a life in the bushes rather than trees. Overall, the anatomy is consistent with either a scansorial or an arboreal preference. Taking the vertebrae and further elements into account: "These convergences strongly suggest Eomaia was an agile animal with climbing skeletal adaptations, capable of grasping and branch walking, and active both on the ground and in trees or shrubs (for example, like the opossum Didephis, some species of the tree shrew Tupaia, and the dormouse Glis)." A weight estimate of 20-25g is given, and this is about the same as a standard mouse.
Was this is the Dawning of the Age of Eutheria...
"On the basis of 268 characters sampled from all major Mesozoic mammal clades and principal eutherian families of the Cretaceous, Eomaia is placed at the root of the eutherian tree with Murtoilestes and Prokennalestes. Clearly, these three taxa are closer to living placentals than to living marsupials. Eomaia is placed in Eutheria by numerous apomorphies in the dentition, the wrist and the ankle", (Ji et al 2002, p.820).
The diversity among basal eutherians suggests some of the clades arose before Eomaia, (p.821). The next oldest known genus is Murtoilestes, and that dates from close to the Barrmian-Aptian transition. The North American Montanalestes (Aptian-Albian) is anatomically distinctive from its Asiatic contemporaries. Eomaia is the earliest eutherian known, but it wasn't the first to have lived. The lineage was established prior to 125 million years ago, and the search for the oldest representative will continue.
Does anyone known the Latin for 'dawn granny'?
The holotype of 'dawn mother' is CAGSO1-IG1 a and b. Both the positive and negative slabs reside at the Chinese Academy of Geological Sciences, Beijing. I'm a fan and this fossil is 'tastic. This is near the start of You in Theria.
Nature chose the week of publication to offer me free access to the full on-line edition. As well as the description, it contained an excellent commentary by Anne Weil, which nicely places Eomaia into perspective. It's in English rather than paleo-speak. Dr Luo then generously forwarded a copy of the publication, along with various other papers. Many thanks.
An equally astounding basal metatherian from Liaoning, Sinodelphys, was described in December, 2003.
|Reference:||Ji et al (2002), The earliest known eutherian mammal. Nature (416), p.816-822.|
Dinosaurier.org, Stefan Jacobasch
A report in German. I'm sure English language links will follow shortly. (Good prediction.)
National Geographic News, 24.4.2002
Earliest Known Ancestor of Placental Mammals Discovered.
Climbing Dawn-Mother, Daniel Bensen
A gentle picture with some thoughtfully written comments.
post-gazette.com, Photo Journal
A nice gallery of pics. The photo showing the fossil in the hands of Drs Luo and Wible, (don't drop it!), gives an excellent impression of the size. You can click on any of the images to view an enlargement.
Eomaia scansoria, Carnegie Museum of Natural History
A collection of various articles, including a link to the original publication, 4.25.02.
|Genus: Eutrochodon Roth, 1903|
|Species:||Eutrochodon sp. Roth, 1903|
|Remarks:||The Roth collection in the Museum of La Plata was reviewed by Simpson in 1936. I doubt this is of any relevance to Mesozoic mammals.|
|Genus: Helioseus Sudre J, 1979|
|Species:||Helioseus insolitus Sudre J, 1979|
|Remarks:||Presumably hard to place for whatever reason.|
Idiogenomys Ostrander GE, 1983|
|Species:||Idiogenomys ozziei Ostrander GE, 1983|
|Place:||Raben Ranch, Nebraska|
|Age:||Chadronian, Lower Oligocene (or Upper Eocene)|
|Reference:||Ostrander (1983), New Early Oligocene (Chadronian) mammals from the Raben Ranch local fauna, Northwest Nebraska. J. of Paleontology 57, p.128-139.|
Evolution of Tertiary Mammals of North America
The publisher's blurb for a major study from 1998. Any book priced at 180 pounds must surely qualify as a major study.
Juramaia Luo Z-X, Yuan C-X, Meng Q-J & Ji Q, 2011|
The following scribblings have been prompted by the arrival of Luo et al, 2011, and thanks go
to the kindly supplier.
|Species:||Juramaia sinensis Luo Z-X, Yuan C-X, Meng Q-J & Ji Q, 2011|
|Place:||Daxigou locality, Tiaojishan Formation, Liaoning|
|Age:||Oxfordian, Upper Jurassic|
|Remarks:||The following is based on my
reading of Luo et al, 2011. With thanks to the supplier. There are bound to be overlaps with
the text immediately above.|
This here Juramaia is the debut contribution from a new mammal producing locality called Daxigou, and what a performance for the first bow. True, everything beneath the thirteenth rib is unavailable for interviews, but it does still manage to present impressions left by ancient hairs.
Looking at various press reports that heralded this description, readers could become charmingly misled into believing this 160 million year old 'Jurassic mum' was literally both a shrew and a rodent! Surprisingly enough, some journalists don't appear to be all too conversant with the ins and outs of the classification of contemporary mammals, let alone that of our Mesozoic relatives. What on Earth do they teach in journalist college if not such basics?
Pittsburgh's Post-Gazette (Museum curator discovers earliest placental mammal, 27.8.2011) avoided those errors although, despite the headline and subsequent mentions in the text, it also wasn't a placental. It's more akin to being a eutherian forerunner of we placentals, the most ancient yet to be discovered. The report included a couple of details that appealed. Firstly, Zhe-Xi Luo confided that he: "wasn't too impressed" upon first inspecting this ancient corpse in its then under-prepared form. That clearly changed once obscuring bits of stone had been removed so as to provide a fuller picture. Secondly, significant research was apparently conducted in his hotel room. So far this year I've been afforded hospitality by two different hotels in Offenbach, and neither provided quite this kind of room service. Yes, the small bottles of shampoo and fluffy towels were very welcome but, surely, many guests would enjoy few things more than finding the front half of a small, long dead mammal at their bedside.
To pay fair due to journalists, should such a thing be legal, as well as not necessarily being on the ball with the intricacies of mammal systematics, they're also confronted with a readership that might think 'Eutheria' is some obscure ritual of the Catholic church. Seen in that light, using 'placental' isn't such a bad approximation. And the media certainly caught an important element of the story. Far from being not too impressive, when the fossil had been cleaned up it was a wow-whoozer. As an indication of the unusual level of interest, I first learned of this publication by flicking innocently through the BBC teletext pages whilst on holiday in England. Teletext doesn't normally include information on Mesozoic mammals.
A true 'true beast'
Generally, if you were to hand an appropriate paleontologist half a Mesozoic mammal skeleton, then they'd be joyful, misty-eyed and temporarily speechless in astonishment, gratitude and awe at such a privilege and treasure. However, a couple of localities in the province of Liaoning yield skeletons of exceptional completeness. True, in this case, the accompaniment of hair impressions was a neat touch (p.442), but even that's not novel in Liaoning. Nevertheless, Juramaia is certainly new news. At one shove it pushes the known fossil record of Eutheria back an astounding 35 million years.
It's this combination of age (apparently, 160 million years) and its affinities.
Like you, me and a dog named Boo, 'Jurassic mummy' is undoubtedly a eutherian; a 'true beast'. For example, should you wish to look at its lower molars from side view, then you'll surely see their overall profile is strongly reminiscent of the teeth of Eomaia. They're akin to a row of kingly thrones, with their backs set towards the front of the mouth and their talonids providing the seats. And the distinction in form between the morphology of the premolars and molars is very pronounced.
The postcanine dental formula is typical of early eutherians. Per side are located: (uppers): 5 incisors, 1 canine, 5 premolars and 3 molars; (lowers): 4, 1, 5 and 3 respectively. This contrasts with the basal metatherian count of 3 premolars and 4 molars. The upper right premolar team actually includes six members. The third in the row is a milk tooth with the adult P3 erupting through behind. That also runs counter to the metatherian habit of ultra-reduced tooth replacement. They only replace the final premolar of a row, and none of those to its fore.
Finer details of molar construction are also 'true beast'-ly and not metatherian. For example, crests on the upper molars, the postmetacrista and the postprotocrista, provide shearing mechanisms for slicing up yummy food with the help of the lower molar ahead. This is much more strongly developed than would be metatherian norms. With regards to the lower molar talonid, cusps termed the hypoconulid and entoconid aren't close together, as for Sinodelphys, or twinned up, as with more derived metatherians. The lower jaw has a naturally occurring hole, a mental foramen, positioned below where premolars number 4 and 5 meet. For metatherians, that final mental foramen is further back below the first molar.
A southern radiation of mammals, the australosphenidans, had apparently independently developed molar teeth capable of both crushing and slicing food; functionally tribosphenic. However, in contrast to those critters, this 'Jurassic mum' has no trace of a postdentary trough towards the rear of the internal side of the lower jaw.
What for a 'true beast'?
Having clearly ruled out metatherian and australosphenidan affinities, and just as clearly demonstrating eutherian credentials, the job became one of seeing if this fossil could be fitted into any already established lower taxon. In this case, given the age of 35 million years older than any other known 'true beast', that could be seen as impossible or, to be more cautious, merely vanishingly improbable. But comparisons still need to be made.
The upper canine of this critter is double-rooted, a difference to the single-rooted model worn by Eomaia. In contrast to Acristatherium, 'Jurassic mum' has differing numbers of upper and lower incisors, the upper M3 molar is larger, and there are no gaps present between the foremost upper premolars (p.443). The list continues so as to rule out this critter being fitted into any already established Lower Cretaceous genus.
Juramaia was scored for two independently compiled matrices of taxa and morphological characters. It was found to be a closer relative of living placentals than any Cretaceous metatherians but, also not surprisingly, one of the basal-most known eutherians. Its anatomy is very old fashioned for a 'true beast' although, in some regards, perhaps less so than the later Acristatherium.
Assuming the age assessment of 160 million years stands up to subsequent testing, then this would be too late for the most recent common ancestor of myself and marsupials. That critter must've lived earlier, and very conceivably prior to the Upper Jurassic. This is more in line with recent estimates based on genetic molecular research which, prior to this identification, appeared in stark contrast with the dates of known fossils. That gap is now closed to a considerable degree. Such molecular clocks appear to tick more reliably for mammals than I'd given credit for. For example, one cited recent estimate (p.444) pointed to that divergence beginning at between 143 back to 178 million years ago. As my expectations had been inclined towards the more recent end of that scale, I appear to have been wrong.
Climb for your life!
'Jurassic mum's teeth indicate a healthy interest in wrecking the health of insects, its body size points to a weight of around 16 grammes, and front limb and paw features pronounce good climbing abilities; perhaps, but not necessarily, a life with plenty of time in the trees. I find the final paragraph of the description interesting to think about (p.445, with only reference numbers removed).
"The earliest-known eutherians Juramaia and Eomaia and the earliest metatherian Sinodelphys are scansorial mammals, and differ from contemporary Mesozoic mammals, most which are terrestrial... This suggests that the phylogenetic split of eutherians and metatherians and their earliest evolution are accompanied by major ecomorphological diversification, notably scansorial adaptation, which made it possible for therians to exploit arboreal niches."
The authors neither say nor imply that a life in the trees was a complete novelty for mammals. Volaticotherium, a possibly Middle Jurassic non-therian (indeed, in no way remotely close to Theria) gliding mammal, was very obviously strongly arboreal in its habits. A good case could also be made for Henkelotherium from the Upper Jrassic of Portugal being a tree-dweller. While being much closer to therians than Volatico was, it's also not a member of that gang. Therians can't have been the only mammals in them there trees.
(BMNH) PM1143, a partial skull and half a skeleton with its complete teeth, is one of the star attractions of the Beijing Museum of Natural History. The specific name refers to China. I confess. When I first saw the catalogue number, I was wondering how on Earth this specimen had arrived in London's Natural History Museum. They typically use BMNH. It took me a day to notice the word Beijing!
|Reference:||Luo et al (2011), A Jurassic eutherian mammal and divergence of marsupials and placentals, Nature 476, p.442-445.|
Montanalestes Cifelli RL, 1999|
|Species:||Montanalestes keeblerorum (Cifelli RL, 1999) Cifelli RL, 2000|
|Aka:||Montanalestes keebleri Cifelli RL, 1999|
|Place:||Cloverly Formation, Montana|
|Age:||Albian-Aptian, Lower Cretaceous|
Much of the following is derived from my reading of Cifelli,
This genus is based on associated, partial lower jaws. The right one contained at least four premolars (though the number when alive is unclear) and three molars. "Differs from primitive Tribosphenida (for example, Kermackia), in so far as is known, in having a semimolarized last premolar (lingually placed paraconid, incipient talonid basin, metaconid present) and only three molars; molars differ in having a broader, more fully basined talonid with three cusps consistently present", (p.363). The most comparable known taxon seems to be Prokennalestes from the Upper Cretaceous of Mongolia. Prokennalestes, a definite member of eutheria, is more or less completely known from a number of skulls and skeletons. Montanalestes thus appears to belong within (or very near to) Eutheria.
A relatively derived feature of the dentary is the absence of a Meckelian groove, (p.364). In contrast, a roughened depression on the bone could indicate the continuing presence of a coronoid; a small, 'extra' jaw bone. If so, this would be a basal characteristic.
At the time of publication, Cifelli didn't quite place this genus within Eutheria. Rather, (p.365): "If Montanalestes is related to Eutheria, as the balance of evidence indicates, then the continent of origin for the group is called into question; evidently, Eutheria enjoyed a far broader Early Cretaceous distribution (echoing that of the triconodont mammal Gobiconodon) and an earlier evolutionary radiation than previously envisaged."
Subsequent publications might suggest that the author is psychic. In 2002, an undoubted and earlier eutherian was described, (Eomaia from Liaoning in China). Adding to the portrayal of prescience is a further Liaoning species first published in July, 2003; a new species of the aforementioned Gobiconodon. Clearly, basal representatives of the line which led to Placentalia were scampering around Asia at least 125 million years ago, (something like 15 million years earlier than this genus). As eutherians were around and the known remains of Montanalestes look eutherian, that's most probably what they represent. Ask Luo et al 2002, p.11.
The critter had an estimated body length of ca. 6cm. As the species name is actually in honour of the Keebler family, it was later adjusted in accordance with Latin grammar. There’s possibly a second, unnamed species. The holotype, in the Oklahoma Museum of Natural History, is OMNH 60793.
|Reference:||Cifelli (1999), Tribosphenic mammal from the North American Early Cretaceous. Nature, 401, p.363-366.|
Richard Cifelli, Nature 401
Includes the first few paragraphs of the article in pdf format.
The Shawnee News-Star Online, 28.9.99
A reader-friendly report for non-specialists. Intelligent journalism.
Neodesmostylus Khomenko, 1927|
|Species:||Neodesmostylus sp. Khomenko, 1927|
|Remarks:||I don't know what this genus is hanging around here for either. The age indicates the last ice-age. Old Desmostylus was from the Miocene and was "probably related to Sirenia and Proboscidea, but not a member of either as far as is known." (With thanks to David Marjanovic.)|
Obtususdon Xu Q, 1977|
|Species:||Obtususdon hanhuaensis Xu Q, 1977|
|Place:||Qianshan Basin, Dou-mu Formation, Anhui Province|
|Age:||Nongshanian, Upper Paleocene|
|Remarks:||This seems to fit somewhere within the order Notoungulata. A cast of the holotype is in the Peabody collection.|
|Reference:||Xu (1977), Two new genera of old Ungulata from the Paleocene of Qianshan Basin, Anhui. Vertebrata Palasiatica 15(2), p.119-125.|
Order Notungulata, Martin Jehle
Plesiorycteropus Filhol, 1895|
'near the aardvark'Aka: Hypogeomys G Grandidier, 1912 (partly); Majoria ('for Major') Thomas, 1915; Myoryctes Forsyth Major, 1908
Remarks: This is a hard-to-place but recent fossil genus. It has been considered as
aardvark-like, but such similarities might be a matter of convergence. It appears it was
some kind of digger, with vaguely basal hangovers. The order Bibymalagasia has been
proposed. Though irrelevant to the project in hand, it sounds quite interesting.
Reference: Forsyth Major (1908), A giant sub-fossil rat from Madagascar, Myoryctes rapeto, gen et sp nov. Geol Mag V (5), p.97-98, figs. 1, 2.
Abstract for Bulletin 220, AMNH
|Species:||Plesiorycteropus madagascariensis Filhol, 1895|
|Aka:||Majoria; Myoryctes rapeto; Hypogeomys boulei Grandidier, 1912 (partly)|
A burrower the size of a small dog, this creature probably had a taste for ants, etc. The
wildlife of the island suffered a fair few extinctions about a thousand years ago. This
genus was one of the victims.|
MNHNP 328 is a partial skull of probably a juvenile or subadult discovered by M. Grevé in November, 1894 or possibly earlier. It struts its stuff in the collection of the Museum National d'Histoire Naturelle, Paris. The specific might just have some geographical connection, but this is merely my guess. The specimen was collected somewhere near Belo in the middle of the west coast.
|References:||Filhol H (1895), Observations concernant les mammiferes contemporains des Aepyornis a Madagascar, Bull. Mus. Hist. Nat., Paris, 1, p.12-14.|
|Forsyth Major, CI (1908), A giant sub-fossil rat from Madagascar, Myoryctes rapeto, Geol. Mag. (series 5), 5, p.97-98.|
|Grandidier, G (1912), Un nouvelle espece subfossile d'Hypogeomys, 1'H. Boulei, G. G. Bull. Mus. Nat. Hist. Nat., Paris, 18, p.10-11.|
A brief, comprehensible description of a definitely non-Mesozoic mammal.
|Species:||Plesiorycteropus germainepetterae MacPhee RD, 1994|
|Remarks:||I've got a copy of MacPhee, 1994 but, as
the subject is somewhat beyond the scope of my core interests, I doubt I'll get
around to giving the paper the attention it'd would require. Even the abstract on
page six is long, as abstracts go. MacPhee addresses proposed affinities with
the aardvark (Orycteropus), and accepts there are similar
derived characters. However, these also tend to be
shared with other digging mammals such as anteaters. They're adaptations for
pursuing the same sort of trade, and can be blamed on convergence rather than
shared ancestry. "Accordingly, the view that Plesiorycteropus is
unambiguously aardvarklike in its morphology and adaptations is not supported in this
Jumping forward to page 34 allows me to mention MNHNP 327, the type fossil resident at the Museum National d'Histoire Naturelle, Paris. It's a partial adult skull, and the specific name honours Dr Germaine Petter from the MNHNP. A complete femur was allocated to this species.
As well as being somewhat smaller, the new species differs in some details of the skull. One particularly significant characteristic is shared; extinction.
Size and bye bye
A series of calculations came up with body weight estimates ranging from between around 6 to 12 kilos. The variation results from comparions with differently built animals.
The time of extinction of this genus is considered on page 159. Presumably, as no mention of anything of this sort appears in early chronicles, the animal was deader than the dodo prior to Europeans arriving in Madagascar. That would suggest before around 1650. A popularly cited figure for this extinction is about a thousand years ago. It's easy enough to remember, and roughly coincides with the end of an extinction wave that beset the wildlife of the island.
|Reference:||MacPhee RD (1994), Morphology, adaptations and relationships of Plesiorycteropus and a diagnosis of a new order of eutherian mammals, Bulletin of the American Museum of Natural History, 220, p.1-214.|
Prodelttheridium Trofimov BA, 1984|
Remarks: This generic name wasn't in connection with a formal description. This means it's a nomen nudum. The spelling of the name seems rather odd. I've taken it from Wible et al 2001, p.16. The material is still undescribed.
|Species:||Prodelttheridium kalandadzei Trofimov BA, 1984|
|Age:||Albian-Aptian, Lower Cretaceous|
"In light of the name, this form was presumably thought to
have affinities with the basal metatherian Deltatheridium (Gregory & Simpson, 1926;
Rougier et al., 1998) and, therefore, to occupy a position between Vincelestes and
Late Cretaceous eutherians", (Wible et al 2001, p.16).|
The formulation chosen places this between V. and Late Cret eutherians, which means it could represent a non-eutherian. A formal description would help.
|Reference:||Trofimov in Reshetov & Trofimov (1984), Review and study of fossil mammals from the USSR. In Sokolov & Kucheruk (eds), Theriology in the USSR, p.6-29.|
|Genus: Telacodon Marsh OC, 1892|
|Species:||Telacodon laevis Marsh OC, 1892|
|Place:||Lance Formation, Wyoming|
|Age:||Maastrichtian, Upper Cretaceous|
|Remarks:||Simpson, 1951 provides some information. As with Batodon in the same study (p.9), Marsh established this genus for lower jaw material preserving premolars but not molars. Telacodon had the shorter canine. Several specimens, including the type fossil, are at the Peabody, Yale.|
|Reference:||Marsh (1892), Discover of Cretaceous Mammalia, Part III, American Journal of Science, 43, p.249-262.|
NAFMS, Lull's Locality 5
|Species:||Telacodon praestans Marsh OC, 1892|
|Place:||Lance Formation, Wyoming|
|Age:||Maastrichtian, Upper Cretaceous|
|Remarks:||This species is briefly discussed in Simpson,
This was based on part of a lower jaw with two molars. However, as it's probably metatherian, it can't be sensibly regarded as belonging to this genus. I've got no idea about current thinking on this fossil, assuming there is some. Simpson relates a similar fate befell a fossil referred by Marsh to Batodon tenuis.
|Reference:||Marsh (1892), Discover of Cretaceous Mammalia, Part III, American Journal of Science, 43, p.249-262.|
|Genus: Tingamarra Godthelp H, Archer M, Cifelli RL, Hand SJ & Gilkeson CF, 1992|
|Species:||Tingamarra porterorum Godthelp et al, 1992|
|Age:||Upper Paleocene or Lower Eocene|
|Remarks:||This was described as a condylarth of
sorts, a proto-ungulate. If correct, it's fifty million years earlier than terrestrial
placentals were supposed to have reached Australia. However, it's affinities now seem less
than clear, (Kemp 2005, p.209). Although it lacks some typical characteristics, others have
interpreted it as a marsupial; either an ameridelphian
or a specialised australidelphian. In contrast to typical marsups, the entoconid and
hypoconulid of the molar talonid aren't twinned.|
Tingamarra's based on a single tooth, though other possible bones have been identified.
A certain eutherian in the same fauna is Australonycteris clarkae Hand, Novacek, Godthelp & Archer 1994. This is among the oldest records of a bat I've heard of.
Tingamarra's the name of the property, while Murgon is the township. The age was originally considered to be Lower Eocene, but radiometric dating has subsequently shown the fossils are possibly somewhat older, (with thanks to Brian Choo for the update).
|Reference:||Godthelp et al (1992), Earliest known Australian Tertiary mammal fauna. Nature (London) 356, p.514-516.|
Australia's Lost Kingdoms
An accessibly written summary.
A look at the fossil site in Queesland. Quite a place.
Cadbury Yowie and the Lost Kingdoms
Paleontology meets chocolate.
|Genus: Veratalpa Ameghino, 1905|
|Species:||Veratalpa sp. Ameghino, 1905|
Wanotherium Tang & Yan, 1976|
|Species:||Wanotherium xuanchengensis Tang & Yan, 1976|
|Place:||Xuancheng Basin, Anhui Province|
|Remarks:||A couple of cast specimens are at Yale.|
Bissekty Formation and Aitym Formations, Kyzylkum Desert, Uzbekistan
An upper molar, with anatomical affinities to Daulestes, has been recovered from the Aitym Formation, (upper Turonian - Coniacian). It's about 50% larger than those of D. nessovi. Larger molars have also been reported from the somewhat older Bissekty Formation, and these seem to imply the presence of two or more species in that fauna. Studies continue. (Averianov & Archibald 2003, p.6-7.)
Some notes on Taxonomy and Zoological Classification|
The alternative title is What do we mean by species, genus and family? A bit of background and history on classification.
Should anybody have any further information, I'd be pleased to hear of it.
Regarding references and Bibliography:
With thanks to all the featured sources.
Trevor Dykes, January 2002. Latest up-date: 10.9.2011
|With further thanks due to:|
The Prehistoric Data Files
Martin Jehle, Paleocene mammals of the world, Cohort Placentalia
An excellent reference. Some Mesozoic remains feature.
BIOSIS, The Index to Organism Names
The Society of Vertebrate Paleontology BFV Online, (John Damuth)
The Peabody On-line VP Catalogue
Dr John Alroy, North American Fossil Mammal Systematics Database
Weight guestimates and nomenclatural details.
David Marjanovic, for suggestions, corrections and lessons in Russian grammar.
Averianov AO & Archibald JD (2003), Mammals from the Upper Cretaceous Aitym Formation, Kyzylkum Desert, Uzbekistan. Cretaceous Research 00 (2003), p.1-21.
Bülow K von (1921), Erdgeschichtliche Zeiträume, Eine geologische Umschau, (in) Kosmos Handweiser für Naturfreunde 1921, Heft 6, Seiten 141-145.
Cifelli RL (1999), Tribosphenic mammal from the North American Early Cretaceous. Nature, 401, p.363-366.
Fox RC, Scott CS & Bryant HN (2007), A new, unusual therian mammal from the Upper Cretaceous of Saskatchewan, Canada, Cretaceous Research, 'Article in Press' edition, p.1-9.
Hu Y, Meng J, Li C & Wang Y (2010), New basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China, Proceedings of the Royal Society B, 277, p.229-236.
Ji Q, Luo ZX, Yuan C-X, Wible JR, Zhang JP & Georgi JA (2002), The earliest known eutherian mammal, Nature 416, p.816-822.
Kemp TS (2005), The Origin and Evolution of Mammals, Oxford University Press, pp.331.
Kordikova ZB, Polly PD, Alifanov VA, Rocek Z, Gunnell GF & Averianov AO (2001), Late Cretaceous and early Tertiary microvertebrates from the North Eastern Aral Sea Region of Kazakhstan, Journal of Paleontology, 75 (2), p.390-400.
Luo Z-X, Yuan C-X, Meng Q-J & Ji Q (2011), A Jurassic eutherian mammal and divergence of marsupials and placentals, Nature 476, p.442-445.
MacPhee RD (1994), Morphology, adaptations and relationships of Plesiorycteropus and a diagnosis of a new order of eutherian mammals, Bulletin of the American Museum of Natural History, 220, p.1-214.
McKenna MC & Bell SK (1997), Classification of Mammals Above the Species Level. Columbia University Press.
Reilly SM & White TD (2003), Hypaxial Motor Patterns and the Function of Epipubic Bones in Primitive Mammals, Scienec 299, p.400-402.
Simpson GG (1951), American Cretaceous insectivores, American Museum Novitates, 1541, p.1-19.
Weil A (2002), Mammalian evolution: Upwards and onwards, Nature 416, p.798-799.
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, p.1-144.
Wible JR, Rougier GW, Novacek MJ & McKenna MC (2001), Earliest Eutherian Ear Region: A Petrosal Referred to Prokennalestes from the Early Cretaceous of Mongolia. American Museum Novitates 3322, p.1-44.