MARGINALIA
A Palaeontological Puzzle Solved?
Keith Thomson
The Oldest Vertebrate Larva
One of the difficulties about Palaeospondylus has always
been that obvious vertebrate features such as the dermal skeleton
are missing. However, Palaeospondylus also lacks the teeth,
proper jaws and all the gill apparatus behind the first arch. It has
rudiments of the limb girdles, but no fins. We also have to explain
why Palaeospondylus specimens do not become progressively
more similar to adult lungfish as they become bigger (an early
argument against larval status).
All this makes sense if Palaeospondylus is a true larva
with a sharp metamorphosis in the life history, rather than the sort
of juvenile that slowly morphs into an adult. As a true larval
stage, it would require special adaptations—to compensate, for
example, for the absence of teeth and jaws. Among those is,
obviously enough, the anterior attachment organ. We may then
suggest, although we cannot prove, that Palaeospondylus was
a larval form spending its life in shallow waters, attached to
either a substrate or a plant. Like larval Lepisosteus (and
modern larval lungfish where the attachment organ is on the throat),
it lived partially from the reserves in the yolk sac, and partially
as a detritus feeder on the attached surfaces.
There is only one species of adult lungfish in the Achanarras
beds—Dipterus valenciennesi, a handsome fish
reaching to 30 centimeters or more—but it is never found
smaller than about 60 millimeters. Somewhere there must exist very
young Dipterus fossils or very old Palaeospondylus
that would settle the question. Or are there yet other explanations
still waiting to be explored? The anterior attachment organ might be
consistent with Palaeospondylus being a parasite. In which
case it would not represent the oldest known fossil vertebrate
larva, but the oldest known vertebrate parasite. Or even both. One
of the things that keeps paleontologists (and palaeontologists)
going is the possibility that just around the corner is a deposit
with some superb new material that will answer all our prayers. Or
start another controversy.
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