|
Pikaia
Pikaia was found in the the
Burgess Shale in British Columbia, Canada, of Middle
Cambrian
age. Several features of Pikaia give indications of its
affinity to chordates, though it has the superficial appearance of a
worm. It appears to have cartilage rods supporting what is believed to
be a tail fin and appears to have a notochord. In its body form and its
overall anatomy it seems similar to lancelets of the present, though at
the time of writing it has not been studied in detail, resulting its
evolutionary position remaining enigmatic.
The discovery of this animal demonstrates that animals similar to Pikaia had evolved by
at least 525 Ma.
Though it could be an early, possibly basal, deuterostome (an organism
with 2 openings, that includes all chordates, as well as 3 other phyla)
it was previously classified as a cephalochordate. Pikaia
is another fossil that has been referred to as "a squashed slug" kind
that has been reported to have a notochord.
Long reports that after examining all the main fossils of this animal
Philippe Janvier of Paris has said
he cannot confirm that any of them had any chordate features, suggesting
Pikaia could have been closely related to Yunnanozoon.
Pikaia gracilens (Walcott) is a chordate
from the Cambrian
but no detailed description of it has been available prior to the study
by Morris & Caron (Morris & Caron, 2012)3.
According to the authors3 they studied 114 specimens that
were available, concluding that it was indeed a chordate, and went on to
explore its relationships to chordates, extinct and extant, as well as
other deuterostomes. Pikaia has a fusiform
body that is laterally compressed and has about 100 myomeres. The
bilobed small head has 2 narrow tentacles and there is no evidence of
eyes, and with the exception of a thin dorsal fin, with no fin rays, and
a series of appendages that are bilaterally arranged with possible
pharyngeal pores at the anterior end, and there are no other external
features. As well as the musculature the internal anatomy includes an
alimentary canal that has a prominent lenticular unit that is present
almost always preserved in positive relief. The mouth is implied to be
terminal as the cavity is interpreted as pharyngeal. The anus appears to
be terminal, though the posterior extension of the gut is unclear. A
reflectively preserved unit is the most prominent structure, that they
suggest is possibly hollow, that has been termed the dorsal organ. It
was formerly interpreted as a notochord, they suggest it is less likely
to be a notochord because of its position and size. They suggest it
could have formed a storage organ, though its original function is
uncertain.
There is a narrow stand of tissue,
interpreted a being the nerve chord and notochord, that is ventral to
the dorsal organ. The authors3 also suggest there is evidence
of a vascular system, that includes a ventral blood vessel.
The authors3 say the
presence of sigmoidal myomeres is largely the basis for the position of
Pikaia in the chordates, and in many other
respects the more tentative identification of a notochord,
Pikaia differs from what is expected for the nature of
primitive chordates, as occurs in such animals as amphioxus and the
fossil record of the Cambrian, such as Cathaymyrus,
Haikouichthys, Metaspriggina, Myllokunmingia, and Zhongxiniscus.
The authors3 say they prefer
a scenario regarding Pikaia as the most
stemward of the chordates, linked to the yunnanozoans, that are
controversial, though there is the possibility that Pikaia
is convergent on the chordates. There are implications for evolution of
notochord, gills and myomeres. The wealth of Pikaia
material indicates that the consistency of preservational details
allowed a reconstruction of the anatomy that is reliable, and does not
significantly erode phylogenetically relevant characters, though by
definition there must be some sort of taphonomic imprint.
Sources & Further reading
- Benton, Michael J., 2005, Vertebrate Palaeontology, 3rd
Ed. , Blackwell Science.
- Long, John A.,
2011, The Rise of Fishes - 500 Million years of Evolution,
2nd ed, University of New South Wales Press.
-
Conway, Simon Conway and Caron, Jean-Bernard, 4 March 2012,
Pikaia gracilens Walcott, a stem-group chordate
from the Middle Cambrian of British Columbia, Biological Reviews.
Links
-
Pikaia images
-
Human origins traced to a worm
-
Pikaia-like chordate from the Lower Cambrian of China
|
|