The Rise and Fall of the Multi-cusped

The extinction event at the end of the Cretaceous is often seen as something of a changing of the guard: the old Mesozoic fauna was swept away, leaving the world open for the (eventual) rise of our modern Caenozoic fauna. However, not every lineage that crossed the Cretaceous boundary remains with us today.

Skull of Kamptobaatar kuczynskii in lateral view, without lower jaw. From Kielan-Jaworowska (1971).

Multituberculates (multis to their friends) were a group of small, vaguely rat-like mammaliaforms that first appeared in the Middle Jurassic (but see comments later). The name 'Multituberculata' refers to their most distinctive character: the presence of a particularly large number of tubercles (cusps) on the post-canine teeth, in particular on the fourth premolars and first molars of the upper jaw. The first upper molars of Kamptobaatar kuczynskii, for instance, had ten cusps, arranged in two rows of five (Kielan-Jaworowska 1971). These multi-cusped teeth were faced on the lower jaw by a massively enlarged fourth premolar, which had become high and multi-ridged, like some sort of dental buzz-saw. These massive teeth, together with the loss of the canines in all but a few basal species (Kielan-Jaworowska & Hurum 2001), have been interpreted by most authors as indicating a diet of tough plant matter, like that of most modern rodents, though some small animals such as insects may have also been eaten. As such, multis are commonly regarded as the first mammaliaforms to show obvious adaptations for a vegetarian diet.

Lower jaw of Kamptobaatar kuczynskii, showing reduced dentition between incisor and enlarged fourth premolar. From Kielan-Jaworowska (1971).

For the most part, multituberculates are known from Eurasia and North America, though Hahnodon taqueti is known from the Lower Cretaceous of Morocco, and potential multituberculate teeth have been identified from the Late Cretaceous of South America (Kielan-Jaworowska & Hurum 2001). The Late Cretaceous Gondwanatheria may also represent South American relatives of the multituberculates (Gurovich & Beck 2009). A number of lineages of multis passed through the end of the Cretaceous apparently unscathed, and multituberculates remained a part of the Holarctic fauna up until the end of the Eocene. Their final extinction is usually attributed to competition from the increasingly diverse placentals, particularly the rodents (Kielan-Jaworowska et al. 2004). This may have been related to their reproductive biology: multituberculates had extremely narrow and rigid pelvic girdles that could not have spread much during birth, and they must have produced extremely small young in a similar manner to modern marsupials (Kielan-Jaworowska et al. 2004).

Reconstruction of the North American Palaeocene multituberculate Ptilodus, from Cox (1998) via here.

Some of you may have noticed that I have referred to multituberculates as 'mammaliaforms' rather than 'mammals': they were definitely the former, but it is more debatable whether they are the latter. Most modern authors restrict the name Mammalia to the mammal crown-group: that is, the smallest possible group including monotremes, marsupials and placentals. The question of where multis sit in relation to the mammalian crown-group requires me to mention another group of animals, the Haramiyida, that lived in the Late Triassic and early Jurassic. Haramiyidans have teeth similar to the distinctive arrangement of multituberculates, leading to the assumption when they were first described that they represented early multis (Kielan-Jaworowska & Hurum 2001). However, while multituberculates share a number of features of the non-dental skeleton (such as ear ossicles independent of the jaw) with modern mammals, haramiyidans have an overall anatomy of a much more plesiomorphic grade. If multituberculates are related to haramiyidans, as indicated by their teeth, they would be well outside the mammalian crown group, and would have evolved a number of convergent features independently of mammals. On the other hand, if one goes by the evidence of features other than the teeth, multis were closely related to mammals (either their sister group [Gurovich & Beck 2009] or within the mammal crown, closer to marsupials and placentals than to monotremes [Luo et al. 2002]) and haramiyidans represented an entirely independent acquisition of a multituberculate-like dentition. At present, an independent origin of multis and haramiyidans seems more likely, but it could easily be upset by future discoveries.

REFERENCES

Gurovich, Y., & R. Beck. 2009. The phylogenetic affinities of the enigmatic mammalian clade Gondwanatheria. Journal of Mammalian Evolution 16: 25-49.

Kielan-Jaworowska, Z. 1971. Skull structure and affinities of the Multituberculata. Acta Palaeontologica Polonica 25; 1-41, pls 1-5.

Kielan-Jaworowska, Z., R. Cifelli & Z.-X. Luo. 2004. Mammals from the Age of Dinosaurs: origins, evolution, and structure. Cambridge University Press.

Kielan-Jaworowska, Z., & J. H. Hurum. 2001. Phylogeny and systematics of multituberculate mammals. Palaeontology 44 (3): 389-429.

Luo, Z.-X., Z. Kielan-Jaworowska & R. L. Cifelli. 2002. In quest for a phylogeny of Mesozoic mammals. Acta Palaeontologica Polonica 47: 1-78.

Insectivores: Possibility of Puggles (Taxon of the Week: Australosphenida)

A baby echidna or puggle. Normally, the puggle would be contained in a pouch on its mother's underside. Photo from here.

The Australosphenida is a group of mammals that has been studied fairly extensively in recent years, which is not bad going when one considers that, at most, less than twenty species have been assigned to it and some authors are of the opinion that the majority of those should not be regarded as australosphenidans at all.

The undoubted Australosphenida (or, more correctly if dealing with the restricted grouping, Ausktribosphenida) are five small Mesozoic insectivores (Rougier et al., 2007) - Asfaltomylos patagonicus and Henosferus molus from Jurassic South America, Ambondro mahabo from Jurassic Madagascar, and Ausktribosphenos nyktos and Bishops whitmorei from Cretaceous Australia. Despite their probably being fairly unprepossesing animals in life (as far as we can tell - so far, ausktribosphenids are only known from teeth and jaw bones), ausktribosphenids have provoked a fair amount of interest because of the resemblance between their teeth and those of modern marsupials and placentals. All three groups possess an arrangement called the tribosphenic molar, in which the lower molars each have a large posterior depression that contacts with a large cusp in the corresponding position on an upper molar, facilitating the grinding of food ("like a mortar and pestle", is the comparison that has been used in print).


Evolution of the tribosphenic molar as presented in Luo et al. (2001). Steropodon is an early monotreme; Northern Hemisphere tribosphenids are the clade marked "Boreosphenidans".

The discovery of tribosphenid mammals in Gondwana earlier than they had been found in northern continents (where they appear in the early Cretaceous) therefore led to the suggestion that tribosphenid mammals may have evolved in the Southern Hemisphere and only later spread to the North (modern marsupials, despite their current Southern Hemisphere distribution, were derived from Northern Hemisphere Mesozoic ancestors). However, further phylogenetic analyses lead to the alternative suggestion (Luo et al., 2001) that ausktribosphenids evolved the tribosphenic molar independently from Northern Hemisphere tribosphenids. Instead, Luo et al. (2001) placed ausktribosphenids as related to modern monotremes, which lack tribosphenid molars but share with ausktribosphenids a distinct shelf (the cingulum) around the front of the molars. This ausktribosphenid + monotreme grouping is what Luo et al. (2001) dubbed the Australosphenida. Later analyses (e.g. Rougier et al., 2007) make the 'ausktribosphenids' paraphyletic with regard to monotremes. Alternatively, some analyses have continued to support a monophyletic tribosphenid clade uniting ausktribosphenids, marsupials and placentals that excludes monotremes (Rowe et al., 2008). Things are not made easier by the point that, while ausktribosphenids are known from little else than teeth, known monotremes, both living and fossil, mostly have teeth that are vestigial, absent or just plain wierd (Kollikodon, I'm looking at you).


Kollikodon ritchiei. This Cretaceous monotreme had strangely rounded molars (it has been informally referred to as "Hotcrossbunodon") that may have been used for crushing molluscs. Or they may have been used for something else entirely.

Living monotremes are, of course, restricted to Australia, though it wasn't always so - Monotrematum sudamericum is a monotreme known from the Palaeocene of South America. Other known fossil genera are all Australian. I won't bore you with the things everybody already knows about monotremes - the presence of venomous ankle spurs in platypuses, the four-headed penis of echidnas, or the fact that baby echidnas (which are held in a pouch on the mother's underside) are known as puggles. Some things I will mention - if you've never seen a live echidna, they're a lot bigger than you think they are (I don't know how big you think they are, but I can assure you that they're bigger). According to Wikipedia, Tachyglossus aculeatus, the short-beaked echidna (the species found in mainland Australia), reaches about a foot and a half in length, while the New Guinean Zaglossus species are even bigger. The extinct mainland Australian species known as 'Zaglossus' hacketti (probably not a Zaglossus, but unrevised) would have been as large as a sheep. Echidnas when disturbed are able to dig with their fore-feet in such a way that they effectively sink into the ground while remaining horizontal, meaning that they retain full protection from their spines. In some areas (best known on Kangaroo Island in South Australia; see here) echidnas may form trains - shortly before a female becomes sexually receptive, a train of up to ten males will begin to follow her around in single file, waiting for her to give them the go-ahead*. Echidnas mate lying on their sides dug into a trench made by the male.

*If humans were to do this, it would be regarded as creepy. Echidnas don't seem to have this problem.

And in case you were wondering, I have been informed that the best way to deal with the spines when cooking an echidna is to roast it whole with the spines still on; after it's finished cooking, the spines can be pulled out fairly easily.

REFERENCES

Luo, Z.-X., R. L. Cifelli & Z. Kielan-Jaworowska. 2001. Dual origin of tribosphenic mammals. Nature 409: 53-57.

Rougier, G. W., A. G. Martinelli, A. M. Forasiepi & M. J. Novacek. 2007. New Jurassic mammals from Patagonia, Argentina: a reappraisal of australosphenidan morphology and interrelationships. American Museum Novitates 3566: 1-54.

Rowe, T., T. H. Rich, P. Vickers-Rich, M. Springer & M. O. Woodburne. 2008. The oldest platypus and its bearing on divergence timing of the platypus and echidna clades. Proceedings of the National Academy of Sciences of the USA 105 (4): 1238-1242.

The Mysterious Name of Queen Lestoros

The shrew-opossum Caenolestes fuliginosus. Photo from here.

Those of you who are familiar with the more encyclopaedically-arranged natural history books will almost certainly have encountered the phenomenon of the Mysterious Name. In the introductory section of the book, where the scope of the text is indicated, there'll be some sort of taxonomic listing - the phyla of animals, for instance, or the families of birds - with each of the taxa listing being described in a subsequent part of the book. But often, if you're the sort that will pore over such a listing closely enough, you'll notice that the listing includes at least one name, one taxon (often more) on which the remainder of the book is silent. It's there in the beginning, it has its place firmly indicated in the hierarchy - and then silence.

One taxon that throughout my youth remained to me a mysterious name was the Caenolestidae. Caenolestids are small South American marsupials, commonly known as shrew-opossums*. In most lists of marsupial families, they'll be near the beginning, after the true opossums of the Didelphidae. But all the books I read as a child would skip straight from Didelphidae to Dasyuridae, with nary a hint of anything in between.

*Another sign of their obscurity in the public eye - that they are only given the names of other animals, rather than being thought deserving of a name of their own.

Admittedly, the caenolestids are not a large family. Gardner (2005) lists just six species in three genera, Caenolestes, Lestoros and Rhyncholestes. Four of those species are in Caenolestes, the other two genera are regarded by Gardner as monotypic (though one effect of their understudied status is that no two authors will entirely agree on the caenolestid species list, and some authors may recognise two species in either or both of the smaller genera, while others will recognise only a single genus with as few as three species). Of course, that's still more species than other mammal families such as Rhinocerotidae or Hominidae that have no trouble claiming page space for themselves, and while caenolestids may be few in number now, they were more abundant in the past. Caenolestids were the most abundant small marsupials in South America during the early Miocene (Marshall, 1980).


Lestoros inca. Photo by Phil Myers.

Living caenolestids are widespread, and probably not particularly uncommon, but specimens are few and far between. This has mainly been blamed on their unobliging choice of habitat - they prefer very dense, humid forest, though they may be concentrated close to open meadows (Nowak, 1999). They are shrew-like in appearance (hence the common name), and females lack a pouch (presumably the young just hang directly onto the teats, but females with emerged young seem to have not yet been observed). The front of the lower jaw contains an elongate, procumbent pair of incisors, on which more in a moment.

The most detailed account of their behaviour comes from Kirsch & Waller (1979), who trapped and observed specimens of four caenolestid species. Though stomach contents indicate that the caenolestid diet is mostly invertebrates (Nowak, 1999), Kirsch and Waller found that specimens were most attracted to traps baited with meat, and when offered a choice between insects and meat, they would more readily take the latter. A male caenolestid offered newborn rats proved an efficient predator:

The animal would move toward a rat, sniffing vigorously, seize and lift the rat with its forepaws or pin it to the substrate, and bite it several times quickly with its incisors. The caenolestid would then commence eating the rat by biting off a section of the head with its cheek teeth and take successive bites posteriorly.

In fact, the large incisors were used rarely by caenolestids in feeding - almost all biting and chewing was done with the cheek teeth, and the incisors are primarily for dispatching prey. Caenolestids have a distinct flap on either side of the upper lip, and this probably protects the face and whiskers from getting clogged up with blood and dirt while the animal is busy stuffing prey towards the back of its mouth. When offered larger food items such as earthworms, the caenolestids would sit upright on their tails and use their front paws to manipulate their food, similar to the way a mouse does.


Skull of Caenolestes condorensis, so you can get a better look at those lower incisors which a such bad news if you're a baby mouse. Image from here.

Fossil caenolestids (or near-caenolestids, depending on your preferred classification) were ecologically more diverse than modern species, and a number appear to have been herbivorous. One such genus, the Miocene Abderites, had a large sharp and multi-grooved first molar like the teeth of the multituberculates. Marshall (1980), in the last major review of the fossil caenolestids, suggested that the arrival of the caviomorph rodents in South America was what triggered the demise of the caenolestid herbivores, while the more generalised insectivores/carnivores were able to keep sailing on.

Phylogenetically, caenolestids have been difficult. Perhaps the most honest representation of our current state of knowledge of marsupial phylogeny would be a trichotomy between the caenolestids, didelphids and australidelphians (Australian marsupials), with all possible relationships between these three having been suggested in the past. Some earlier authors suggested a relationship between caenolestids and the Australian diprotodont marsupials on the basis of the procumbent incisors, but this hypothesis was pretty firmly flattened when it was established that a different pair of incisors was involved in each of the two groups. Perhaps the most popular option at present is that caenolestids are the sister to australidelphians, to the exclusion of didelphids, as supported by some molecular data (Springer et al., 1998). However, a relationship between didelphids and caenolestids remains a distinct possibility due to the occurence in both of sperm pairing. After leaving the testes, sperm of members of these two families connect up to each other, forming a single moving pair (perhaps enabling them to swim faster through the uterus). However, the homology of this character is debatable, as the sperm connect in a different place in the different families.

REFERENCES

Gardner, A. L. 2005. Order Paucituberculata. In D. E. Wilson & D. M. Reeder (eds.) Mammal Species of the World: A taxonomic and geographic reference pp. 19-20. JHU Press.

Kirsch, J. A. W., & P. F. Waller. 1979. Notes on the trapping and behavior of the Caenolestidae (Marsupialia). Journal of Mammalogy 60 (2): 390-395.

Marshall, L. G. 1980. Systematics of the South American marsupial family Caenolestidae. Fieldiana: Geology, new series 5: 1-145.

Nowak, R. M. 1999. Walker's Mammals of the World, 6th ed. JHU Press.

Springer, M. S., M. Westerman, J. R. Kavanagh, A. Burk, M. O. Woodburne, D. J. Kao & C. Krajewski. 1998. The origin of the Australasian marsupial fauna and the phylogenetic affinities of the enigmatic monito del monte and marsupial mole. Proceedings of the Royal Society of London Series B - Biological Sciences 265 (1413): 2381-2386.

More Mysterious Palaeogene Eutherians

A few weeks ago, I wrote a post about some of the distinct groups of eutherian mammals that waddled through the world during the Palaeocene, the time period that followed directly after the end of the Cretaceous. At the time, many of the modern groups of mammals were either still fairly marginalised or yet to put in an appearance, and the relationships of most of those primordial eutherians such as pantodonts and taeniodonts remains a remarkable mystery. In this post, I thought I'd focus on one of those early groups that seems to get given an even shorter shrift than most (in fact, this post will be unillustrated because my attempts to find suitable free images online drew a complete blank) - the Tillodontia.

Tillodonts are known only from the Palaeocene and Eocene of North America and Eurasia. Most authors have recognised a single family, the Esthonychidae, though Lucas & Schoch (1998) positioned the genera Lofochaius and Basalina as a paraphyletic series outside that family*. They were medium to large herbivores (one of the later genera, Trogosus, may have weighed around 150 kg - Lucas & Schoch, 1998). Like most mammals of the time, these would not have been the most graceful of beasts - they would have probably been built more like a barrel on legs, perfect for the moist, densely-forested conditions of the time. One of the most distinct features of the tillodonts was the development of large, rodent-like incisors, which in one later clade became open-rooted and permanently-growing like those of rodents. The powerful dentition this gave tillodonts, together with the sturdy legs and claws found in those few species for which post-cranial material is known, would have allowed them to tackle some pretty resilient food-sources, and it is easy to imagine them gnawing bark off trees or digging up roots. A similar lifestyle appears to have also characterised another group of Palaeogene herbivores, the taeniodonts, which also developed rodent-like gnawing teeth. It was once suggested on this basis that taeniodonts and tillodonts were closely related to each other, but the gnawing teeth in taeniodonts were the canines, not the incisors, so the two groups could not have possibly shared a common gnawing ancestor.

*The authors of the late Palaeocene Chinese genus Yuesthonyx (Tong et al., 2003) established a new family for it, Yuesthonychidae. Not only would this family be redundant with its single genus, but Rose (2006) implies that Yuesthonyx is a more derived form not far from the origin of the Trogosinae (see below), making the recognition of a separate family for it all the more pointless.

The very earliest tillodonts such as Lofochaius and Meiostylinodon come from the Lower Palaeocene of China, and this would appear to represent the place of origin for the clade (Rose, 2006). The early Chinese genera were much smaller than the later trogosines, and had less exaggerated dentition. The first North American tillodonts make their appearance in the very end of the Palaeocene with the similarly generalised Azygonyx which survived into the beginning of the Eocene alongside Esthonyx, the most common genus of tillodonts. These forms all lacked permanently-growing incisors, the appearance of which marks the appearance of the clade Trogosinae in the Eocene. Trogosines are known from both North America (Tillodon and Trogosus) and China (Higotherium and Chungchienia), so their geographic origins are unclear. The Chinese Chungchienia had the most advanced dentition of any tillodont - not only were the second incisors a whopping 26 cm long(!), but the ever-growing rootless condition of the incisors was extended to the cheek-teeth (Chow et al., 1996), implying that it must have had an exceedingly tough diet.

While it is fairly well-established that tillodonts were not related to taeniodonts, it has been a decidedly more difficult prospect to establish exactly what they are related to. Van Valen (1963) suggested a close relationship to Arctocyonidae, a family of "condylarths", but this was based on comparisons with the relatively derived North American Esthonyx rather than the mostly then-undiscovered Asian genera. More recent authors have suggested a relationship with the pantodonts, with which tillodonts share dilambdodont cheek teeth. Basal tillodonts may also be difficult to distinguish from basal pantodonts (Rose, 2006). The Palaeocene North American Deltatherium may also be relevant to the origin of tillodonts. However, none of these groups has yet been subject to a proper cladistic analysis to determine whether their shared features indicate actual relationship or convergence. And even if these taxa do form a monophyletic clade, this still just takes a number of small problematic clades of unknown relationships to modern taxa and turns them into one big clade of unknown relationships to modern taxa!

REFERENCES

Chow, M., J. Wang & J. Meng. 1996. A new species of Chungchienia (Tillodontia, Mammalia) from the Eocene of Lushi, China. American Museum Novitates 3171: 1-10.

Lucas, S. G., & R. M. Schoch. 1998. Tillodontia. In Evolution of Tertiary Mammals of North America (C. M. Janis, K. M. Scott & L. L. Jacobs, eds.) pp. 268-273. Cambridge University Press.

Rose, K. D. 2006. The Beginning of the Age of Mammals. JHU Press.

Tong Y.-S., Wang J.-W. & Fu J.-J. 2003. Yuesthonyx, a new tillodont (Mammalia) from the Paleocene of Henan. Vertebrata PalAsiatica 41: 55-65.

Van Valen, L. 1963. The origin and status of the mammalian order Tillodontia. Journal of Mammalogy 44 (3): 364-373.

Life in the Palaeocene - We Don't Need No Placentalia?

The pantodont Coryphodon, as reconstructed by Heinrich Harder. At the time of their existence, pantodonts were the largest herbivorous mammals. According to Wikipedia, Coryphodon reached about a metre in height and a weight of half a tonne, and also had the dubious distinction of having the smallest brain/body weight ratio of any mammal living or extinct.

Sixty-five million years ago last Tuesday, the mighty dinosaurs went extinct. Well, they didn't all go extinct, but that's how it's usually expressed because "the mighty dinosaurs went extinct except for a number of volant clades that actually continued to do pretty well for themselves, really" somehow just doesn't have quite the same ring to it. What remains a fact is that something pretty significant happened to the ecosystem at the end of the Cretaceous, leading to a major turnover that's usually represented as out with the dinosaurs, bring in the mammals. It is true that the mammals showed a significant rise in diversity during the Palaeocene, the time period immediately following the Cretaceous. However, few of the prominent mammalian groups of the time would be recognisable today.

Modern mammals are divided between monotreme, marsupials and placentals. It is the Placentalia (the group we ourselves belong to) that have been the most successful of the three groups overall, a success that has generally been attributed to their reproductive system of nourishing developing foetuses for longer periods and giving birth to more developed young*. When the fossil record is actually taken into account, Placentalia are a subset of a larger group called Eutheria. Eutherians are the total group containing placentals and all fossil mammals more closely related to placentals than marsupials, while placentals are the crown group of the eutherian lineages that have survived to the present.

*Whether this is really the secret of the placentals' success is more debatable than generally let on. For instance, it has been suggested that in highly unpredictable environments such as the arid centre of modern Australia, marsupials, with their lower nutrient commitment to developing offspring, may actually have the edge reproductive system-wise.


Skull of the taeniodont Psittacotherium, from Matthew (1937) via Paleocene Mammals. Late Palaeocene taeniodonts developed massively powerful jaws and cutting teeth. Psittacotherium was one of the most extreme forms, and at a weight of about 50 kg would have been comparable in size to a medium dog.

The eutherian and marsupial lineages had separated from each other by the early Cretaceous, but the question of when the modern placentals arose has been a hotly debated topic. While a number of Cretaceous lineages have been suggested to belong to the Cretaceous crown group - Zhelestidae as relatives of the ungulates (hoofed mammals), while Zalambdalestidae were close to rodents and lagomorphs (Archibald et al., 2001) - recent analyses have placed these taxa outside the placental crown, and the fairly comprehensive analysis by Wible et al. (2007) suggested that none of the fossil eutherians known from the Cretaceous are placentals. This stands in fairly stark contrast to molecular dating studies, which are fairly unanimous in suggesting that the modern placental orders diverged from each other during the Cretaceous. Either the molecular dating is all wrong for some reason, or the placentals were around in the Cretaceous and we just haven't found them yet.

Still, whether it was the ancestors of the placentals or a number of lineages that survived the end of the Cretaceous, the fossil evidence indicates at least four eutherian lineages survived into the Palaeocene. The Cimolestidae and Leptictidae, families present in both the Cretaceous and the Palaeocene, were placed by Wible et al. (2007) outside the placentals, while the Taeniodonta, a eutherian lineage of unknown relationships, was represented in the late Cretaceous by the species Schowalteria clemensi (Fox & Naylor, 2003). Whether the various other lineages known from the Palaeocene diverged from these lines after the end of the Cretaceous or also survived from earlier times is a decidedly open question.

As already indicated, few of the Palaeocene eutherians can be related directly to modern placental orders. Instead, the Palaeocene was the time of a number of lineages that are no longer with use - herbivores such as the pantodonts and dinocerates, small insectivores such as apatemyids and leptictids, carnivores such as creodonts and arctocyonids. Martin Jehle's Paleocene Mammals website has detailed coverage of many such groups. Palaeocene mammals were also quite distinct from modern taxa in the overall range of morphologies - for want of a better way to put it, Palaeocene eutherians tend to look - well - lumpier than modern species. The broad grasslands that currently dominate the terrestrial part of the world were not yet in existence, and the Palaeocene was a time of forests. As a result, the grassland-adapted cursorial morphologies like modern horses and antelope were also absent, and the low-slung waddler was king.


The early dinocerate Prodinoceras xinjiangensis, as reconstructed by Stanton Fink.

So how did these Palaeocene waddlers relate to the modern taxa evolutionarily? The only answer we can really give at this point is, who knows? The relationships between the Palaeocene and the modern eutherian orders remain almost completely unknown, and those few connections that have been accepted in the past have been profoundly shaken. For instance, many of the Palaeocene families have been included in the 'condylarths', a heterogeneous assemblage believed to be related to the modern ungulates. However, it has become well established in recent years that the ungulates represent at least three separate lineages, with the artiodactyls (even-toed hoofed mammals), perissodactyls (horses and rhinoceros) and paenungulates (elephants and hyraxes) all arising from separate ancestors in the placental tree. Which condylarths are related to which modern ungulates? For that matter, are they related to any of them? If the ungulate morphology arose at least three times in lineages that survived to the present, why should we assume that it couldn't have also appeared independently in extinct lineages? Similar issues surround Palaeocene 'insectivoran' families, whose association with possibly polyphyletic modern insectivorans should be regarded as doubtful.

In light of the findings of Wible et al. (2007), we might even doubt whether many of the Palaeocene eutherians even represent placentals. The classification of McKenna and Bell (1997) united many early eutherians such as Cimolestidae, Pantodonta and Taeniodonta (as well as the modern pangolins) into a group called Cimolesta, which was then included in the Ferae with creodonts and Carnivora. While pangolins may indeed be related to carnivorans, Cimolestidae, as referred to above, are not even placentals. What then becomes of the rest of the "Cimolesta"? Are they also stem-eutherians like Cimolestidae, or are they true placentals?

Such questions are not mere curiosities - the answer could have significant effects on our understanding of Palaeocene ecology. At least some stem eutherians such as the Zalambdalestidae possessed epipubes, bones that support the pouch in marsupials but are absent from placentals (Kielan-Jaworowska, 1975). Because of the restrictions epipubes place on the expansion of the abdomen, they may be incompatible with a placental reproductive system. As a result, we cannot assume that stem eutherians bore well-developed young like modern placentals do. Did pantodonts walk around with pouches slung from their bellies?

REFERENCES

Archibald, J. D., A. O. Averianov & E. G. Ekdale. 2001. Late Cretaceous relatives of rabbits, rodents, and other extant eutherian mammals. Nature 414: 62-65.

Fox, R. C., & B. G. Naylor. 2003. A Late Cretaceous taeniodont (Eutheria, Mammalia) from Alberta, Canada. Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen 229 (3): 393-420.

Kielan-Jaworowska, Z. 1975. Possible occurrence of marsupial bones in Cretaceous eutherian mammals. Nature 255: 698-699.

Wible, J. R., G. W. Rougier, M. J. Novacek & R. J. Asher. 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary. Nature 447: 1003-1006.

A little bit mole-ish in the Miocene

After the last two posts on unicellular organisms, I'm going to bravely leap to another end of phylospace and cover a mammal. Necrolestes patagonensis was a small fossorial animal from the Miocene of Patagonia that has always held a certain appeal for me, both because of its somewhat morbid genus name (it translates as "robber of the dead") and because of its enigmatic phylogenetic position (recent review by Asher et al., 2007).

Necrolestes was described by Florentino Ameghino in 1891. Ameghino seems to have left a few conundrums in his wake - he originally described the giant carnivorous bird Phorusrhacos (sometimes spelt Phororhacos - I'll have to explain that sometime) as a toothless mammal, and mistakenly described the "wingless" penguin Palaeoapterodytes (see here for an explanation). Ameghino seems to have favoured an association of Necrolestes with the African golden moles (Chrysochloridae) - a not unreasonable suggestion for the time. Since then, probably the majority of authors have felt that Necrolestes was a marsupial, but it has also been compared with edentates or suggested as a late survivor of an equally enigmatic group of South American fossil mammals called Gondwanatheria (see here). I recall seeing a nice little cartoon in one paper doubting a marsupial affinity for Necrolestes (I think it was Archer, 1984 but I'm not certain) showing a little Necrolestes being drop-kicked by an anthropomorphised borhyaenid out the door of a gathering of marsupial representatives (borhyaenids were a family of dog-like marsupial carnivores).

After a detailed redescription of the available material (which, among other things, introduced me to the glorious-sounding term schmelzmuster, which refers to the spatial arrangement of different enamel types within a tooth), Asher et al. (2007) attempt to shed some light on the position of Necrolestes by trying to match its characters with previously optimised trees for other mammals. This proves to be quite tricky - Necrolestes has a rather oddball combination of primitive and derived characters, and any suggested position requires a certain amount of convergence. Asher divide the possibilities into three main options - a position outside the Theria (the marsupials + placentals clade), a position close to or within marsupials (metatherians), and a position close to or within placentals (eutherians).

In regards to a position outside Theria, Necrolestes has an atlas (the first cervical vertebra after the skull) with the left and right halves not fused to each other, something unlike any adult therian. It also lacks many of the tooth apomorphies associated with Theria, though this may just be due to the simplified teeth of Necrolestes. However, Necrolestes does have a coiled cochlea, an astragalar neck and lacks a septomaxilla, so Asher et al. conclude it is most likely a therian. As Gondwanatheria is often regarded as non-therian, Asher et al. suggest that Necrolestes is probably not a gondwanatherian, but I feel that the non-therian nature of Gondwanatheria has not really been demonstrated.

In regards to whether Necrolestes is a metatherian or eutherian, Asher et al. don't really come to a firm conclusion. Patterson (1958) claimed that Necrolestes possessed epipubic bones, which are a primitive character retained in marsupials but absent from modern placentals (thought they were present in some stem eutherians). Asher et al., however, found no sign of epipubic bones. It also has a non-inflected mandibular angle, which is generally a eutherian character, but is also found in some derived marsupials. Necrolestes does share a number of characters with metatherians, most of them "absence" characters - lack of a stapedial artery sulcus, lack of a labial mandibular foramen, etc. It agrees with metatherians in having three premolars, but seems to have one too few molars (three instead of four), and shares a ball-shaped distal process on the ulna and transverse canal foramina on the basisphenoid with crown marsupials.

Characters shared with eutherians are a posteriorly small zygomatic process on the squamosal and small incisive foramina, as well as the aforementioned non-inflected mandible and lack of epipubic bones.

Overall, Asher et al. feel that a metatherian affinity for Necrolestes is most likely, which is appealing on biogeographical grounds (most South American insectivores and such at the time being marsupials). However, they admit that a eutherian affinity can't be ruled out, and I would certainly like to see this possibility further investigated. In particular, if Gondwanatheria are related to edentates (another South American group) as some authors have apparently suggested, the idea that Necrolestes is a late survivor of them may yet be reborn.

REFERENCES

Ameghino, F. 1891. Nuevos restos de mamiferos fosiles descubiertos por Carlos Ameghino en el Eoceno inferior de la Patagonia austral. Especies nuevas adiciones y correcciones. Revista Argentina de Historia Natural 1: 289–328.

Archer, M. 1984. Origins and early radiations of marsupials. In Vertebrate Zoogeography and Evolution in Australasia (M. Archer & G. Clayton, eds.) pp. 585–626. Carlisle: Hesperian Press.

Asher, R. J., I. Horovitz, T. Martin & M. R. Sanchez-Villagra. 2007. Neither a rodent nor a platypus: a reexamination of Necrolestes patagonensis Ameghino. American Museum Novitates 3546: 1-40.

Patterson, B. 1958. Affinities of the Patagonian fossil mammal, Necrolestes. Breviora Museum of Comparative Zoology 94: 1–14.