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Box 6.2 The Irish deer: too big to survive?

The Irish deer Megaloceros, formerly called the Irish elk, is one of the most evocative of the Ice Age mammals (Fig. 6.5a), and one of the most misunderstood. When the first fossils were dug out of the Irish bog, Thomas Molyneux wrote of them in 1697, "Should we compare the fairest buck with the symmetry of this mighty beast, it must certainly fall as much short of its proportions as the smallest young fawn, compared to the largest over-grown buck."

This was a large deer, some 2.1 m tall at the shoulders, and it famously had massive antlers, the largest spanning 3.6 m. The old story was that this deer simply died out because its antlers became too large. Paleontologists understood that the antlers were subject to sexual selection and, as in modern deer, the male with the largest antlers and the scariest display probably gathered the largest harem of females and so passed on his genes most successfully. But can a species really be driven to extinction by sexual selection?

In a classic paper, the young Steve Gould (1974) showed that this was clearly nonsense. He measured the body lengths and antler dimensions of dozens of specimens and showed that they fell precisely on an allometric curve, and that the allometric curve was the same as for other relatives such as the smaller, living red deer and wapiti (Fig. 6.5b). It is clear that sexual selection and natural selection were at odds in this case, as often happens, but the balance was maintained and indeed the Irish deer was successful throughout Europe, existing until 11,000 years ago in Ireland and 8000 years ago in Siberia. It probably died out because of climate change at the end of the Pleistocene and hunting by early humans, rather than by collapsing beneath the weight of its overgrown antlers.

It is worth reading Gould's (1974) classic study of positive allometry in the Irish deer, and a broader review of positive allometry in sexually-selected traits by Kodric-Brown et al. (2006). Read more and see color illustrations at http://www.blackwellpublishing.com/paleobiology/.

Figure 6.5 Positive allometry in the antlers of the giant Irish deer Megaloceros. (a) A famous photograph of an Irish deer skeleton mounted in Dublin in Victorian times. (b) Positive allometry in the antlers of modern deer, showing that Megaloceros (M) falls precisely on the expected trend of its closest living relatives. Note that the fallow deer (D) plots above the slope (i.e. antlers are larger than expected from its height), and the European and American moose (A) plot below the line (i.e. antlers are smaller than expected from their height). Two regression lines, the reduced major axis (steeper) and least squares regression, are shown. The allometric equation is antler length = 0.463 (shoulder height)174. (Based on information in Gould 1974.)

Figure 6.5 Positive allometry in the antlers of the giant Irish deer Megaloceros. (a) A famous photograph of an Irish deer skeleton mounted in Dublin in Victorian times. (b) Positive allometry in the antlers of modern deer, showing that Megaloceros (M) falls precisely on the expected trend of its closest living relatives. Note that the fallow deer (D) plots above the slope (i.e. antlers are larger than expected from its height), and the European and American moose (A) plot below the line (i.e. antlers are smaller than expected from their height). Two regression lines, the reduced major axis (steeper) and least squares regression, are shown. The allometric equation is antler length = 0.463 (shoulder height)174. (Based on information in Gould 1974.)

brain, which again is rather well developed at birth.

Ichthyosaurs (see Figs 6.3, 6.4) were born live underwater, as shown by remarkable fossils (see p. 462), and did not hatch from eggs laid onshore, as is the case with most other marine reptiles. Their large head at birth would have allowed them to feed on fishes and ammonites as soon as they were born. The large eyes were perhaps necessary also for hunting in murky water, and had to be near-adult size from the start. Or, perhaps, it made them look cute and encouraged parental care!

Shape variation between species_

Within any clade there are many forms. Related plants and animals usually show some common aspects of form, and species and genera vary around a theme. For example, gastropods all have coiled shells and the three-dimensional shape can be thought of as a result of variation in four parameters (see p. 333). When form can be reduced to a small number of parameters like this, then the whole range of possible forms governed by those parameters may be defined - the theoretical morphospace for the clade. Studies of the theoretical morphospace for gastropods, ammonoids and early vascular plants show that known species have only exploited a selection of possible morphologies. Some zones of morphospace may represent impossible forms - such as gastropods or ammo-noids with a minute aperture, with no room for the living animal - but others have simply not been exploited by chance, or they cannot be reached by normal evolutionary change because of the impossibility of intervening stages.

The range of forms within a clade may also be described as disparity, the sum of morphological variation. Disparity may be quantified as the range of values for all possible shape parameters seen in species in a clade. All the measures of shape may be combined in a mul-tivariate analysis that can simplify dozens of shape measures to a smaller number of principal coordinates or eigenvectors (see p. 139) so that size and other general principles may be separated. It is possible to compare the disparity of different clades, or to look at how disparity varies through time. Disparity is generally high early in the history of a clade as the species "try out" all the possibilities of their new body form, and then the disparity of the group remains rather constant for the rest of its history. Changes in disparity through time may roughly mimic changes in diversity (as diversity increases, so too does disparity), but the correlation is usually not perfect, and shape change often goes ahead of diversity increase.

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