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| Tunicate larva and tadpole |
The major grouping of animals, or phylum, known as Chordata is of particular interest
because it contains all vertebrate animals (subphylum Vertebrata) as well as provides a historic relationship with a
number of organisms, like starfish, that show no
identifiable similarities with mammals seen in the zoo. What ancestor we have in common with this
invertebrate would have to go back many millions of years. Starfish belong to the phylum Echinodermata. This grouping is made up of sea urchins, sea
cucumbers and other marine animals in addition to the starfish. Their link to us probably goes back well over
five hundred million years, to the early Paleozoic
era – near the beginning of fossils that have been discovered with the unaided
eye.
There is no clue about vertebrate origins when you examine
an adult echinoderm. Their morphology
lacks any of the characteristics we associate with modern vertebrates – such as
a brain and an internal skeleton. The
relationship with vertebrate ancestors is found in the larval stage of animals similar to these. The larva of the sea squirt, the tunicates, has some of the hallmark
characteristics found in Chordates – ancestors to the vertebrates. These include a notochord (which serves as a backbone), a dorsal nerve cord and segmented
muscles – all characteristics found in today’s vertebrate animals.
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| Adult sea squirts |
How juvenile traits are retained by adult descendants is
explained by the process called paedomorphosis. For chordates to descend from the larval form
of some ancestral echinoderm would require the larva’s reproductive organs to
mature prior to reaching the adult stage.
Descendants of this process might retain these larval characteristics
into adulthood if they prove beneficial to the animal’s survival.
The notochord is a flexible, rodlike structure that extends
the length of the body providing the animal an axis for
muscle attachment and giving the animal the undulating movement needed
for propulsion through water. For
vertebrates the notochord appears during embryonic development and becomes the
basis for the vertebrae.
The dorsal nerve cord enables the development of a central nervous system and the enlargement of the
anterior end into what becomes the organism’s brain. This allows for a more sophisticated body
plan based on greater awareness of one’s surroundings and the means to quickly move
to more suitable locations and to pursue prey.
Segmented musculature controlled by a centralized nervous
system improves coordination and to enable a
response time rapid enough to effectively pursue prey or to avoid the lunge of
a predator. These elements, found in the
ancient fossils of the first chordates, are elaborated upon over time to
include a postanal tail – a muscular
appendage extending beyond the anus that significantly improves the animal’s means
of propulsion.
The earliest chordates undoubtedly had pharyngeal pouches that were perforated and would be used for filter feeding.
These slits enabling water to pass through would later be the basis for respiration, gills that enabled the exchange of gases
between the animal and the water. Embryonic
pharyngeal pouches give rise to the Eustachian tube, the middle ear and other
devices in land-based vertebrates.
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| X-Ray Tetra reveals vertebra |
Other chordate characteristics that would be increasingly
elaborated upon over time would include an endoskeleton, paired appendages, a
complete digestive tract and a ventral heart with a closed blood system to go
with it. The body plan worked because it
proved to be extremely adaptable and the
resulting vertebrates were able to widely diversify in
form and function to fit the demands of most of the world’s habitats.
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