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.

Palaeos page on Stenocybus

A new page is up for critique at Palaeos on the basal dinocephalian Stenocybus acidentatus.