All the World's Deer: A History of the Deer Family

Megaloceros

Over the past year, I have described no less than 53 living species of deer, seven of which are currently considered endangered and one of which, bar a few park escapes, exists only in captivity. And that's excluding Schomburgk's deer which. as I noted, probably went extinct in 1938, and certainly didn't last long beyond that if it didn't. It's the only known species of deer to have gone extinct for centuries, although it might not be the last to do so. But there are, unsurprisingly, given their large size and distinctive features, a great number of extinct fossil species of deer that we know about.

Further along in this blog I have posted a summary of the family tree of the living species of deer. This is derived from molecular and genetic studies, but it turns out to map reasonably well to what we would have guessed purely from looking at the animals and their skeletons, which is good news if we want to try and place fossil deer into it. The first thing that's apparent from the tree is that, as predicted back in the 19th century, there is a deep split within it, representing the two subfamilies: the cervines (at the top in the diagram) and the capreolines.

Within the cervine subfamily, the first split is between the muntjacs and what we might call the 'typical cervines'. Currently, the oldest known muntjac is Muntiacus leilaoensis, which lived during the Late Miocene, between 7 and 9 million years ago. We only know of it from its antlers, but, while they had the same distinctive shape, they were larger than those of most modern species. This implies that muntjacs (and the closely related tufted deer) became smaller and reduced their antlers over the course of their evolution. So their apparently primitive appearance is probably nothing of the sort, but rather an adaptation to their habitat, as is also true, outside the deer family, of some of the smaller antelopes.

The typical cervines (technically, the tribe Cervini) must therefore be at least this old. During this time, some of them have also shrunk, producing some similarly muntjac-sized species of deer. But these were species trapped on small islands, examples of insular dwarfism where a limited food supply benefits smaller animals. Examples include the Ryukyu deer (Cervus astylodon), native to a chain of islands south of Japan during the late Ice Ages, and the Cretan dwarf deer (Candiacervus ropalophorus) which lived at around the same time. The latter was the smallest of a number of (probably) closely related deer found on the island and apparently did not undergo the reduction in brain size we might expect from an isolated herbivore with few, if any, natural predators.

But it's the largest prehistoric cervines that are surely the best known. Eucladoceros lived throughout the Pliocene from western Europe to China. Some of the larger species are estimated to have weighed 350 kg (770 lbs), similar to a female moose, but it's their antlers that are really distinctive. One of the later forms, E. ctenoides, had comb-like antlers, with a series of at least four tines branching off the main beam one after the other. It seems to have been primarily a browser. E. dicranios, which lived at around the same time had among the most complex of all antler structures, with each of the series of tines themselves branching into two or even three points, giving a veritable forest of branches.

Perhaps the best-known fossil deer of all, however, is the "Irish elk" (Megaloceros giganteus), likely the largest of the cervine deer. When it was discovered in Ireland, its huge size and palmate antlers suggested that it might be related to moose - known as 'elk' in Europe - and, although they are now known to be found across Europe and into southern Siberia, the name stuck. Fortunately, they lived recently enough that it has been possible to do the sorts of genetic studies on their bones that we normally do on living animals, and there have been enough now to show clearly that the Irish elk was not really a moose, but actually a giant relative of the fallow deer.

The stags of Irish elk, the last in a long line of related animals dating back to at least the Pliocene, stood about 2 metres (6' 6") tall at the shoulder, making them about the same size as the largest living subspecies of moose. They had huge flaring antlers, weighing something like 40 kg (88 lbs) and with an incredible 3.5 metres (11 feet) span; the skull had unusually thick bone, perhaps partly to support them. 

Deer family
(click to enlarge)

That the does were significantly smaller than the males supports the suggestion from other lines of evidence that Irish deer stags competed for mates in the same manner as the larger living deer and had similar herd structures. Although it has been suggested that they were too large to effectively wrestle with, instead being used only for visual display, this has been challenged, although their fighting styles may have been more limited than seen in, say, red deer. The last Irish elk may have died out, on the Isle of Man, as recently as 7,000 BC.

The capreoline deer, now known primarily from the New World, nonetheless originated in Eurasia. The oldest known is probably Procapreolus, which originated in eastern Europe at least 10 million years ago before dispersing elsewhere. It's a possible ancestor for living roe deer which, if true, puts the origin of the capreolines as a whole even further back. 

The oldest known fossil deer in America is Eocoileus, which lived much later than Procapreolus, about 5 million years ago at the Miocene/Pliocene boundary. It's known from Florida and lived at about the time that most estimates suggest deer first entered the continent. Interestingly, the antlers had some resemblance to those of roe deer, rather than to the more complex structures seen on the larger American deer of today.

Another potentially interesting North American species is the Toronto subway deer (Torontoceros) discovered during the excavation of the eponymous transit system. It was a moderately large deer, living 11,000 years ago right at the end of the Ice Age and probably looked rather like a reindeer/caribou, but with larger, horizontally arranged antlers. Its relationships to other capreoline deer are unclear, which leaves open the possibility that it might genuinely be an early relative of reindeer, a branch of the deer family about whose origin we know little. 

Deer must have first entered South America shortly after the Panama Isthmus formed at around the dawn of the Pleistocene epoch. While we do know of a few South American fossils from the time, they are sufficiently incomplete that it's hard to say much about them, although it seems that, as with muntjacs, the brockets and pudu descended from larger ancestors, rather than being truly 'primitive'.

Molecular evidence shows that moose are likely an early branch of the wider capreoline subfamily, but their fossil history dates back only a little over 2 million years with just a single fossil genus, Cervalces, being known. This was of a similar size to the Irish elk, and, by some estimates, slightly larger, making them both contenders for the title of "largest deer ever". Three or four species have been identified, with some debate as to which ones are truly distinct. They lived from Europe to North America throughout the Pleistocene and, despite some differences in the shape of the skull, the rest of the body was already broadly similar to that of living moose, suggesting that more distant ancestors must exist but have not yet been discovered.

The antlers of Cervalces, known as the broad-fronted moose in Europe and the stag-moose in North America (these are probably, but not necessarily, separate species) had unusually long beams with a different shape to the palmation than in living moose, perhaps making them more effective in display, but potentially getting in the way in dense woodland. From what we can tell from where their fossils have been found, they lived in open pine forests and steppeland and had a similar diet to modern moose. Despite the lack of other known contenders, however, it is not certain that they are the direct ancestors of the modern species.

When we come to very early deer it can be difficult to tell which, if either, of the two subfamilies they belonged to. In many cases, this is because we don't have the leg bones that would provide a definitive answer, but many probably don't have living descendants anyway. As a result, some of the earliest known deer fossils are placed in either a third subfamily or simply left outside the subfamily scheme altogether.

Procervulus

In some schemes, the earliest known deer is Procervulus, which lived in Europe during the Early Miocene, perhaps as far back as 20 million years ago. I say "in some schemes" because it isn't universally recognised as being a deer, rather than some close relative. This is partly because its branching horns did not possess the burr that would indicate that they were really antlers and that they were shed on a regular basis. 

Procervulus was followed, first by Heteroprox, and then by Euprox, the latter dying out around 9 million years ago in the Late Miocene. These, and related early species such as Dicroceros, may have been the first deer to leave their place of origin in Europe and head out into Asia. More significantly, Euprox had undisputed antlers, perhaps making it the first deer to do so.

They were, however, somewhat different from the antlers of most modern deer. Instead, the burr was high up rather than next to the skull, with the result that only the tips of the antlers were shed, the bulk remaining as permanent horns. Something similar is seen today in muntjacs, although not to the same extent, and the antlers of these early deer had a different shape, with two tines branching off just above the burr. Also like muntjacs, and some other small modern deer, they had large canine teeth in the upper jaw and probably used those as much as the antlers when competing with one another.

A 2003 study of ruminant relationships placed the split between the deer family and the antelope/musk deer lineage as occurring around 27 million years ago. Whether we would really call those very early animals "deer" is a matter of taste. Lagomeryx, a small Eurasian ruminant that lived on into the Middle Miocene has been suggested as representative of what these proto-deer might have looked like. Indeed, it is sometimes recognised as an early deer itself, rather than a close relative. With the smallest examples of the genus not much larger than a rabbit, and most roughly muntjac-sized, it had tiny horns that nonetheless split into a crown of spikes at the tip and the long canine teeth seen in the earliest definitive deer.

From such humble beginnings the family branched out to give rise to the mighty red deer and long-legged moose that we have today.

[Photos by "Sterilgutassistentin" and "ghedoghedo" from Wikimedia Commons. Cladogram adapted from Heckeberg et al. 2016, Pitra et al. 2004, Gilbert et al. 2006, Zhang et al. 2021, and Srisodsuk et al. 2018.]