Page 162 - Introduction to Paleobiology and The Fossil Record

FOSSIL FORM AND FUNCTION 149

Stylopod Paralogs
9 101112 13
Knockout phenotype Developmental
Phase I expression axis
Phase I • HoxD-9, D-10

Radial

Zeugopod 9 101112 13 Knockout phenotype
Developmental time Phase II • HoxD-9 Phase II expression (b)

• HoxD-9, D-10
• HoxD-9, D-10, D-11
• HoxD-9, D-10, D-11, D-12
• HoxD-9, D-10, D-11, D-12, D-13

Autopod 9 101112 13 (c)
Knockout phenotype
Phase III expression
Phase III • HoxA-13
• HoxA-13, D-13
• HoxA-13, D-13, D-12, D-12, D-11, D-10
(a)

Figure 6.9 Hox genes and the development of the tetrapod limb. (a) The sequence of growth of a
tetrapod limb bud, reading from top to bottom, showing how the stylopod (humerus/femur),
zeugopod (forearm/calf) and autopod (hand/foot) differentiate. The pattern is determined by

turning on (filled squares) and off (open squares) of Hox genes D-9 to D-13. (b, c) Interpretation
of the forelimbs of the osteolepiform fi sh Eusthenopteron (b) and the tetrapod Acanthostega (c)
in terms of development. The developmental axis (solid line) branches radial elements (dashed
lines) in a pre-axial (anterior) direction in both forms, and the digits of tetrapods condense in a
post-axial direction. (a, based on Shubin et al. 1997; b, c, courtesy of Mike Coates.)

soft tissues in sequence from the body outwards to the tips of the fingers. In an osteolepiform fi sh

(Fig. 6.9b), the developmental axis presumably ran through the main bony elements, and additional
bones, radials, developed in front of the axis (pre-axial side). In tetrapods (Fig. 6.9c), the axis in the

leg (arm) runs through the femur (humerus), fibula (ulna) and ankle (wrist) and then swings through

the distal carpals (tarsals). Radials condense pre-axially at first, as in the osteolepiform, forming the
tibia (radius) and various ankle (wrist) bones. The developmental process then switches sides to
sprout digits post-axially (behind the axis). This reversal of limb-bud growth direction in the hand/
foot is matched by a reversal of the expression of the Hox genes. In the zeugopod, HoxD-9 is
expressed in all fi ve zones, HoxD-10 in the posterior four zones, down to HoxD-13 only in the
posterior of the five. In the autopod, on the other hand, HoxA-13 is present in all zones, HoxD-13

in the posterior two zones, and HoxD-10 to HoxD-12 only in the posterior zone.
In Late Devonian tetrapods, six, seven or eight digits were freely produced, and it was only at
the beginning of the Carboniferous that tetrapods seem to have fi xed on fi ve digits fore and aft.
Since then, digital reduction has commonly occurred, down to four (frogs), three (many dinosaurs),
two (cows and sheep) or one (horses) fi ngers and toes. Systematists must beware of interpreting such
events as unique, however: the new evo-devo perspective suggests that loss of digits has happened
many times in tetrapod evolution, and by the same processes of switching Hox genes on and off.
Read more about Hox genes and limb-bud development at http://www.blackwellpublishing.com/
palaeo/, and about evo-devo topics in general in Carroll (2005) and Shubin (2008).