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During the late 1990's, a group of palaeontologists from Northwest
University in Xi'an, China and Cambridge University in England,
discovered two fossils of the earliest vertebrates ever found.
The specimens came from Haikou, 40 km to the west of Chengjiang
in the Yunnan Province of South China. The area around Chengjiang
is becoming internationally recognised as a key locality for
the study of soft-bodied fossils from the
Early Cambrian Period, about 530 million years ago. Led by Prof.
Degan Shu, director of the Early Life Institute at Northwest
University, the group published their discoveries in the international
scientific journal Nature on the 4th November 1999.
The origin of vertebrates, which include mammals, birds, eptiles,
amphibians and fish, is one of the largest mysteries for evolutionary
biology. Clear fossils of the primitive agnathan fish have been
long known from the Lower Ordovician Period (~475 million years
ago), with more questionable examples from the earlier Cambrian
Period (510-545 million years ago). The newly described fossils
from Chengjiang Faunas have been clearly identified as agnathans,
a discovery that pushes back the fossil
record of the vertebrates about 50 million years.
Myllokunmingia is fusiform (i.e. tapered at both ends), 28mm
in length and 6mm in height, with a dorsal fin and a pair of
primitive ventral fins. Several gill pouches are present on its
head. Veins, intestines and zigzag-shaped myomeres can also be
recognised in the fossil.
Haikouichthys is also fusiform, 25 mm in length and 7 mm in
height, bearing a dorsal fin and a pair of primitive ventral
fins. Haikouichthys has fin-rays on its dorsal fin, a feature
that indicates that it is more highly evolved than Myllokunmingia.
Heart, intestines and gonads can also be recognised in the fossil.
Phylogenetic analysis, which uses such physiological features
to classify lifeforms, reveals that the fossils are primitive
fish similar to the living hagfish and lampreys. Each fossil
lacks true bony tissue, but has a pair of ventral fins, consistent
with current theories of the early evolution of vertebrates.
Their existence shows that a variety of vertebrates had already
evolved in the Early Cambrian. The primitive chordates, the group
out of which vertebrates evolved, must have developed
from the more primitive deuterostomes in Ediacaran times, 555
million years ago, if not earlier.
This find is significant not only for palaeontologists; humans
are
vertebrates too. These animals, which are the oldest vertebrates
ever found, must have been closely related to the ancestral species
from which all vertebrates evolved. |
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