218 Evolution? The Fossils Say YES!
FIGURE 9.8. The evolutionary history of the earliest vertebrates, showing the evolutionary position the new
Chinese Cambrian fossils Myllokunmingia and Haikouichthys with respect to other jawless and jawed fish.
(Redrawn by Carl Buell, based on Shu et al. 1999)
HAGFISHMyllokunmingiaLAMPREYHaikouichthysCONODONTSANASPIDSHETEROSTRACANSTHELODONTSOSTEOSTRACANSJAWED VERTEBRATESCAMBRIAN ORDOVICIAN SILURIAN DEVONIAN CARBONIFEROUS PERMIANthe way back to the Early Cambrian, much earlier than previous fossils (which were based
on fragments of dermal armor known from the Late Cambrian). Through the rest of the
Cambrian and Ordovician, we see nothing more in the fossil record of vertebrates other
than isolated plates made of true bone and the microscopic toothlike structures known as
conodonts, so apparently the earliest vertebrates remained small and completely soft-bodied
for some time. Then in the Early Silurian, about 430 million years ago, we find the first nearly
complete armored jawless fish, and this group radiates into an explosion of diversity by the
Late Devonian. This Devonian radiation of jawless fish (fig. 9.1) was spectacular, with a wide
variety of armored fish that still lacked jaws or a muscular bony skeleton, but nonetheless
they were covered in solid bone all over their bodies; some had large curved head shields