Page 158 - Introduction to Paleobiology and The Fossil Record

FOSSIL FORM AND FUNCTION 145

passes through the major nodes of the clado- of heterochronic change, pedomorphosis
gram of vertebrates. The synapomorphies (see (“juvenile formation”), or sexual maturity in
p. 130) of vertebrates appear fi rst, then those a juvenile body, and peramorphosis (“overde-
of tetrapods, then those of amniotes, then velopment”), where sexual maturity occurs
those of mammals, of primates, and of the relatively late. These changes can each occur
species Homo sapiens last. in three ways, by variation in timing of the
Three other aspects of development throw beginning of body growth, the timing of
light on phylogeny. Certain developmental sexual maturation or the rate of morphologi-
abnormalities called atavisms, or throw- cal development (Table 6.1).
backs, show former stages of evolution, such
as human babies with small tails or excessive
hair, or horses with extra side toes (Fig. 6.6a),
showing how earlier horses had fi ve, four or
three toes, compared to the modern one.
Vestigial structures tell similar phylogenetic
stories. These are structures retained in living
organisms that have no clear function, and
may simply be there because they represent
something that was once used. So, modern
whales have, deep within their bodies, small
bones in the hip region that are remnants
of their hindlegs (Fig. 6.6b). Whales last
had functioning hindlegs over 50 Ma in the
Eocene, and the vestigial remnants are still
there, even though they serve no further (a)
purpose in locomotion, and only support
some muscles associated with the penis. femur pelvis
The third aspect of development that forms
links with phylogeny is the observation that
ontogenetic patterns themselves have evolved.
In particular the timing and rate of develop-
mental events has varied between ancestors (b)
and descendants, often with profound effects. Figure 6.6 Hints of ancestry in modern animals.
This phenomenon is termed heterochrony. (a) Extra toes in a horse, an example of an
atavistic abnormality in development, or a
throw-back, to earlier horses which had more
Heterochrony: are human adults juvenile apes?
than one toe; normal horse leg (left), extra toes
Heterochrony means “different time”, and (right). (b) The vestigial hip girdle and hindlimb
includes all aspects of changes of timing and of a whale; the rudimentary limb is the rudiment
rates of development. There are two forms of a hindlimb that functioned 50 Ma.

Table 6.1 The processes of heterochrony: differences in the relative timing and rates of development.
Onset of growth Sexual maturation Rate of morphological development
Pedomorphosis
Progenesis – Early –
Neoteny – – Reduced
Postdisplacement Delayed – –
Peramorphosis
Hypermorphosis – Delayed –
Acceleration – – Increased
Predisplacement Early – –