Page 159 - Introduction to Paleobiology and The Fossil Record

146 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Rec.
Ple.
Miocene Pliocene N. nigricans T. doederleini

T. thomsoni

Oligocene N. antipoda

T. coelata

Paleocene Eocene ontogeny T. squamosa
pedomorphocline

T. boongeroodaensis
juvenile
adult

Figure 6.7 Heterochronic evolution in the Cenozoic brachiopods Tegulorhynchia and Notosaria.
Adults of more recent species are like juveniles of the ancestor. Hence, pedomorphosis (“juvenile
formation”) is expressed in this example. (Based on McNamara 1976.)

In studying heterochrony, it is necessary to sis, and hyperomorphosis in particular (devel-
have a robust phylogeny of the organisms opmental time is longer, but rate of morpho-
in question, an adequate fossil record of logical development is not faster). Thus,
the group, and a sound set of ontogenetic heterochronic changes can occur in different
sequences for each species. This allows the directions in different characters, a phenome-
paleontologist to compare juveniles and adults non called mosaic evolution.
throughout the phylogeny. A classic example In a classic study, McNamara (1976) sug-
is human evolution. It seems obvious that gested that species of the Cenozoic brachio-
human adults look like juvenile apes, with pod Tegulorhynchia evolved into Notosaria
their flat faces, large brains and lack of body by a process of heterochrony (Fig. 6.7). The

hair. These would imply a pedomorphic main changes were a narrowing of the shell,
change in humans with respect to the human/ a reduction in the number of ribs in the shell
ape ancestor. However, other characters do ornament, a smoothing of the lower margin,
not fi t this pattern. For example, developmen- and an enlargement of the pedicle foramen
tal time in humans is far longer than in apes (the opening through which a fl eshy stalk
and ancestral forms, a feature of peramorpho- attaches the animal to a rock). These changes