Sunday, 17 September 2017

Swimming With Dinosaurs

At the highest level, all mammals alive today can be placed into one of three broad taxonomic groups. Around 94% of the known species are placental mammals, a vast group that includes everything from bats to dolphins and from anteaters to humans. Virtually all the others are marsupials, with the strange egg-laying monotremes representing just a handful of species.

But there were once others, many of them existing before placentals and marsupials (and presumably monotremes, although we don't have much in the way of fossils for them) came into existence. Technically speaking, this includes a few side-branches from the two man lines that arose before the last common ancestor of the living forms - for example, creatures close to the line that eventually gave rise to the placentals, but that arose early enough that we can't be sure that they literally had a placenta. But even once we trim out those, there are still quite a few left.

Sunday, 10 September 2017

Pinnipeds: Ross and Weddell Seals

Weddell seal
The majority of 'true' seal species live in the Northern Hemisphere, with just five found south of the equator. While they were once more widespread, today, all five of these live in the extreme south, with the most widespread - the southern elephant seal - reaching Tasmania, southern New Zealand, and Patagonia. The remaining four, which are all relatively closely related to one another, do not range so far north and are exclusively Antarctic and sub-Antarctic.

The Weddell seal (Leptonychotes weddellii) is a typical example. With fully-gorwn males up to 3 metres (10 feet) in length, and weighing about half a ton, they are noticeably smaller than elephant seals, but still pretty large by the standards of seals in the north. They were first discovered during the expeditions of navigator James Weddell, who, in the 1820s, sailed further south than anyone had previously travelled, into the sea that now also bears his name. Since the seals are named for him, rather than for the body of water where they were first found, it's perhaps unsurprising that they are not unique to that sea, and are equally common right round the frozen continent. During the winter, they can travel as far north as South Georgia and other islands of the extreme South Atlantic, but they don't normally reach (for example) the Cape Horn at the southern tip of South America.

Sunday, 3 September 2017

When the Hippos Changed

The hippopotamus family contains just two living species, including, of course, the well-known common hippo (Hippopotamus amphibius). As is so often the case, though, the family has a long fossil history including a number of other species and genera, although, while the fossil species were more widespread than the living forms are today, it's fair to say that it was never a huge or diverse group in the way that, say, antelopes are.

Among the fossil species, several are very closely related to the living common hippo, including both the stalk-eyed hippo (H. gorgops), which probably weighed over 3 tonnes, and the pig-sized Maltese hippo (H. melitensis). Quite where the living pygmy hippo (Choeropsis liberiensis) fits in the fossil family tree is much less clear, but there are a number of species that aren't particularly close to either of the surviving forms.

Taking a broad view of the fossil history of the family, then, palaeontologists have tended to group the hippos into two subfamilies. One are the "hippopotamines", a group of broadly "modern" hippos that includes both of the living species. At least two other genera are also considered to belong to this group, one of which, Hexaprotodon, lived everywhere from Madagascar to Spain and Indonesia, taking in much of northern Africa and southern Asia on the way. Most of these lived during the Pleistocene and Pliocene epochs, which is to say, the Ice Ages and the epoch immediately preceding them. The earliest forms, including the other genus, Archaeopotamus of Kenya and Arabia, lived during the late Miocene, first appearing somewhere around 8 million years ago.