Evolution What the Fossils Say and Why it Matters

210 Evolution? The Fossils Say YES!

(the coelom) of the Deuterostomata forms from an outpocketing of the inner layer of cells,
or endoderm, rather than from a split in the middle layer of cells, or mesoderm, as occurs
in worms and arthropods.
All of these unique specializations show that the echinoderm and chordate larval pat-
tern is the common link between these two very different phyla of animals. And in the past
20 years, every molecular system that has been examined confirms that the Deuterostomata
is a natural monophyletic group, so there’s no longer any doubt among biologists that sea
stars and sea urchins are among our close relatives. From the common larval pattern, one set
of developmental commands begins to produce the larvae of echinoderms, and another set
of pathways produces the classic chordate embryo. We would not expect to find these fragile
embryos preserved in the fossil record, but just a few years ago, a remarkable discovery of
late Precambrian embryos was found in Doushantuo, China, which seem to show that our
earliest common ancestors lived around 600–700 million years ago (as the molecular clock
estimates also suggest).
What defines a chordate besides the notochord? At one end of the basic chordate body
plan (fig. 9.2) are the sensory organs (eyes, nostrils) and a mouth opening into a throat cav-
ity known as the pharynx. In humans, the pharynx houses the vocal cords, but in fish the
pharynx is the region of the gills and gill basket, and in some groups, feeding takes place
in the pharynx as well. Chordates are also distinctive in that they have a nerve cord along
the back, above the notochord, and the digestive tract along the belly, below the notochord.
By contrast, annelids and arthropods have their digestive tract along their backs and their
main nerve cord along their bellies. Finally, many chordates have a long row of segmented
V-shaped muscles known as myomeres that pull and flex the notochord and allow the side-
to-side swimming motion found in nearly all fish. Last but not least, chordates differ from
worms in that the digestive tract ends with an anus not at the very end of the body (as in
worms and arthropods), but only partway back; the tail (composed of notochord and myo-
meres) usually extends behind the anus.

FIGURE 9.2. The basic organization of the chordate body plan. The front part of the body has the sense organs
(eyes, nose) and a mouth with a pharyngeal basket for filtering out food and oxygen. The nerve cord and
notochord run along the back, while the digestive tract runs along the belly. The anus is not at the rear tip of
the body, but midway down the body, with a long tail and V-shaped segmented muscles (myomeres) running
the length of the body. (After Romer 1959; reprinted with permission of the University of Chicago Press)

EyeBrain

Internal

ear

Spinal cord

Esophagus

NotochordGonad

Nephric

duct

Oviduct in female

Kidney Ureter

Cloaca

Intestine

Coelomic cavity

StomachPancreas Spleen

Bile duct

Gallbladder

Heart in

pericardial cavity

LungLiver

Pharynx with

gill slits

Mouth

Nose