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Growing up (and out, and sidways, and around)...

You've grown since you were born.  I know this is nothing new to you.  Something you may not have thought of though, is that you've also changed shape as you grew.  Think of the proportions of a new born.  They're kind of all head, and by that I mean that their heads are a large portion of their full body height.  Actually, a baby is about 3 baby heads tall.  Adults have a smaller head relative to the rest of their bodies; an adult is about 7 adult heads high.  So the proportion of head to body changes as you grow.  Babies and adults are both still 'human-shaped', but that proportional change is what I mean by changing shape.  This change in proportions comes about because of differences in growth rate - the rest of our bodies grow faster than our heads as we get older (until everything stops growing in our late teens).

A growth sequence of humans drawn to the same size.  Notice the change in proportions as we grow.  Also notice how creepy a baby looks when blown up to adult size...

The term for this change in shape during growth is called allometric growth.  The opposite of this is isometric growth - that is growth without a change in shape.  This is like putting something on a copier and scaling it up as you copy it.  Each copy will be bigger, but will have the same shape.  Different parts of a body can grow allometrically or isometrically.  To go back to humans for a second, our bodies grow allometrically, i.e., change proportions as we grow (heads get relatively smaller, legs get relatively longer).  The head itself grows mainly isometrically - an adult head is mostly just a scaled up baby head (this isn't completely true - chins develop as we age, and ours eyes stay about the same size as the head grows around them).

It turns out that allometric growth can actually be seen in the fossil record.  When we have enough individuals of the same species that are different ages, we can see these growth patterns.  A recent paper (open access) by Andrew Farke, of the Alf Museum in southern California, and his colleagues, investigates allometric growth in an duck-billed dinosaur.  Some species of duck-billed dinosaurs are known for their outrageous head-gear - their heads are topped with all manner of tubes and crests.  These decorations are thought to have been used in mating or in species recognition.  Not only were they visually striking, some were probably used for making noises as well.  Researchers have built models of these structures and blown air through them to see what sounds might have filled Mesozoic landscapes.

What Farke and his colleagues studied was how this head-gear grows, or more specifically, how their head changes shape as it grows.  It turns out that the younger individuals don't have the huge head-gear of the adults.  The study illustrates a growth series of individuals (i.e., individuals of different ages) showing how they acquire head-gear as they grow.  This makes intuitive sense if we assume that the head-gear is for mating.  Young individuals don't mate, so there is no need to put energy into building and maintaining the metabolically costly head-gear until they are going to use it.  Human go through similar changes as we go through puberty - axillary (arm-pit) and pubic hair develop, voices deepen in men, mammary glands increase in size in women.  So the growth of head-gear in hadrosaurs is part of their puberty.

Growth series of two duck-billed dinosaurs (from Farke, et al., 2013)

This is a nice straightforward case of growth, allometry and sexual maturity.  When things get really interesting is when there is a change in growth pattern in a descendant from an ancestral form.  Take, for instance, sea squirts, also called tunicates or salps.  These are sea creatures shaped vaguely like a barrel, with a hole on one side for letting water in, and a hole in the top for shooting water out.  They are usual sessile, that is attached to a substrate without moving. The adults don't look much like the vertebrates we know and love, but it turns out that they are probably close relatives of backboned animals, and the key is in their larval stage.  The planktonic larva of tunicates look vaguely like vertebrates.  They have a number of vertebrate characters, like a notochord and segmented muscles, that get lost as they mature and settle into their sedentary lifestyle.  It is thought that evolution from a tunicate-like relative may have involved a form like the larval tunicates.  In this scenario, the larval forms became sexual mature and never settled down, and were the primitive form of later, more active vertebrates.

A model of the evolution of vertebrates from a tunicate (aka ascidian) ancestor.

This type of evolutionary change is called heterochrony, literally "different timing".  In this case it's a specific kind of heterochrony called paedomorphy, "child shape", because the adults of the descendants look like the sexually immature individuals of the ancestor.  There's evidence that these same sorts of changes occurred in our own ancestry.  If you compare the shape of the face of an infant chimpanzee to that of an infant human they are remarkably similar, certainly more so than the adults.  It's only relatively recently that we have begun to appreciate how these developmental changes relate to evolution, and already we are even beginning to understand the genetic changes behind these morphological shifts.  This is one of those great areas where paleontology and modern genetics are going hand-in-hand.

Baby chimp (top) and adult chimp (bottom).  Notice how the shape of the head of the baby is more similar to human heads than the adult.
Posted: 12/23/2013 1:28:22 AM by host | with 0 comments
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