Page 443 - Introduction to Paleobiology and The Fossil Record

430 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 16.1 The world’s oldest vertebrates

There was a sensation in 1999 when Shu Degan and colleagues (Shu et al. 1999) announced a new
fossil vertebrate, Myllokunmingia, from the Early Cambrian locality Chengjiang in China. This site
has become celebrated for the exceptional preservation of all kinds of animal fossils, and it rivals
the Burgess Shale (see p. 249) as a window into Cambrian life. Until 1999, the oldest vertebrates
were much debated, with some tentative Middle and Late Cambrian candidates, but nothing really
certain until the Ordovician.
Myllokunmingia (Fig. 16.2) is tiny, less than 30 mm long – you could hold a hundred or so of

them, like a handful of wriggling whitebait. The head is poorly defined, but there seems to be a
mouth at one end. Relatives seem to show detail in the head, possible eyes and a brain. If it has
such differentiated head features, it is a vertebrate. Behind the “head” are six gill pouches, a possible
heart cavity and a gut. Above these are the notochord, a key chordate character (see p. 410),
and myotomes or V-shaped muscle blocks. There is a narrow dorsal fi n along the back, and possibly

a ventral fi n below. Myllokunmingia presumably swam by flicking its body and fins from side to

side and wriggling forward through the water. None of the Chinese specimens have mineralized
bone – but this does not rule them out as vertebrates. Evidently, the vertebrate skeleton began
as a cartilaginous structure in early forms, and became mineralized with apatite later in the
Cambrian.
In the same paper, Shu et al. (1999) also named Haikouichthys, a similar early vertebrate from
Chengjiang. A rival team, Hou et al. (2002), suggest that Haikouichthys was the same as Myllokun-
mingia, although Shu and colleagues disagree. The two groups, led by Shu and Hou, also disagree

over the identification of different organs within these fossils, and this affects where they are placed
in the vertebrate phylogeny. The Chengjiang fossils are preserved in grey or yellow sediment, and
the fossils may be grey or reddish, with the internal organs picked out in grey, brown and black
colors. Interpreting these multicolored blobs and squiggles would test the patience of a saint, and
yet it is remarkable that such details have been preserved for 500 Myr. There are now more than
500 specimens of these early vertebrates, so further intensive study may clarify their anatomy
further.
See http://www.blackwellpublishing.com/paleobiology/ for relevant web links.

Myotomes Notochord Dorsal fin

Gill pouch Mouth
5 mm Gut Ventrolateral fin ?Heart cavity
(a) (b)

Figure 16.2 The basal vertebrate Myllokunmingia from the Early Cambrian of Chengjiang,
China: (a) photograph of specimen, and (b) interpretive drawing showing possible identities of the
internal organs. (Courtesy of Shu Degan.)