Mammal Diversity Database



Zebu cattle (Bos taurus indicus)
Agricultural Research Service (USDA) Public Domain

How many species of mammals are there? That is the appropriate title of an article (OA here) accompanying the launch of the American Society of Mammalogy's new Mammal Diversity Database (here).


The answer is 6495 species and counting. Compared to Wilson & Reeder Mammal Species of the World (MSW3), which has its own database (here), that is an increase of 1250 in 13 years.


The authors of ASM MDD have erred in favour of the splitters. In particular, acceptance of the taxonomy of Groves & Grubb for Artiodactyla and Perissodactyla (previous post) will raise eyebrows. In anticipation they write, "inclusion of the taxonomy of Groves and Grubb (2011) in the MDD ensures that these taxa are vetted by the greater mammalogical community."


I note, however, that unlike Castello (previous post), they list the zebu as a subspecies of Bos taurus. Important because this species is a focus in research on bovine placentation and reproduction in places like Brazil.


I found the new MDD more difficult to navigate than MSW3. For example, if you enter a higher taxon than genus you get all the entries in that taxon rather than any definition of the Family or Order itself.


As far as I can see there is no possibility to search for synonyms as on MSW3. I have found this search function of MSW3 enormously valuable when delving into the older literature on placentation. Similarly there is little information about subspecies names.


Some choices of terminology meet my approval. They drop Cetartiodactyla in favour of Artiodactyla (whales and their kin are nested within even toed ungulates). Inevitably they retain Afrosoricida for the order that includes tenrecs, otter shrews and golden moles (a really unfortunate name as African shrews are numerous and belong to an entirely different clade).


Most regrettaby, however, they retain Infraclass Placentalia for eutherians. This unfortunate term was introduced by McKenna & Bell and I have ranted about it before (previous post). It has caused countless confusion and the vernacular "placentals" or even "placental mammals" can mislead the unwary to believe marsupials are without placentation. Prothero among them (previous post).

Evolution of altriciality

Kangaroo Joey inside the pouch
Photo by Geoff Shaw, Zoology, Melbourne, Australia CC BY-SA 3.0)

Marsupials and so-called placental mammals share a common ancestor yet pursue very different reproductive strategies. This led Ingmar Werneberg and colleagues to ask what reproduction was like in the common ancestor of marsupials and placentals (here).
They sampled data from the literature and examined specimens in the Hill and Hubrecht Collections at Museum für Naturkunde in Berlin. (In published sources they found examination of photographs and drawings more reliable than verbal descriptions.) There is a huge data set in the Supporting Information.


Altricial and precocial neonates exemplified by the
mouse (above) and guinea pig

It was estimated that the last common ancestor of placental mammals had a gestation of around 4 months and a litter of 3-5 young. The newborn were altricial and had closed eyes and almost naked skin at birth. The precocial lifestyle of hoofed mammals and the guinea pig is a derived feature (previous post). The newborn of marsupials are, of course, highly altricial.

A reconstruction was attempted of the neonate of the therian ancestor. This suggested it was anatomically more like extant placentals than extant marsupials. In contrast, the preweaning period was very long compared to intrauterine gestation and thus infancy in this ancestor was more marsupial-like. The relative timing of eye opening was between that of placental and marsupial mammals. Gestation lasted 31 days in this hypothetical ancestor. It was reduced to 21 days in the marsupial lineage while increasing fourfold in the placental lineage.



Placentalia - a subgroup of placental mammals

Sir Richard Owen (1804-1892)

The current literature abounds with the term "placental mammals;" it often appears in a paper's title. Even though marsupials have a yolk sac placenta - and several a chorioallantoic placenta - they are not included in the definition of Placentalia. This is a constant source of irritation to marsupial biologists and a pitfall for those wishing to stay their friends (see between Scylla and Charibdis).

Placentalia was relaunched by McKenna and Bell in their influential volume Classification of Mammals above the Species Level (New York 1997). It is attributed to Sir Richard Owen (pictured above).

Owen 1837 [sic] page 903

This is the passage cited by McKenna and Bell. It may well be the first usage of Placentalia but is far from being a definition.

In its current usage, Placentalia defines a crown group of mammals comprising extant species and their ancestors. It is not the same as Eutheria, which includes lineages that do not have living descendants. Placentalia therefore is popular in molecular phylogenetics and phylogenomics. The reason is evident: when working with DNA sequences from living species, inferences can be drawn only about character evolution in the crown group.

When looking for the Owen reference I found that McKenna and Bell had got the date wrong. Robert B. Todd commenced publication of The Cyclopaedia of Anatomy and Physiology in 1837 but Owen's article is in volume 4 part 2 published 1849-52 (full reference below).

Owen R. Teeth. In Todd R.B. (Ed.) The Cyclopaedia of Anatomy and Physiology Vol. 4 Part 2, pp. 864-935. London: Longman, Brown, Green, Longmans and Roberts, 1849-1852. McKenna and Bell Refer to page 903.