A Palaeontological Puzzle Solved?
The possibility that Palaeospondylus was a larval form had
always been rejected because of the well-formed vertebral column and
what seemed to be a good bony skeleton. A larval skeleton should
have been cartilaginous and therefore would not have survived.
Unfortunately, our new sections confirmed that, as Sollas had
already discovered, no histological details are preserved in the
bituminized material (although, significantly, cellular details are
preserved in the bones of other, definitely adult, fishes from
Achanarras). But several Palaeospondylus specimens show
evident cartilage and even connective tissue in the curious
apparatus at the front of the head. Some retain an outline of the
body. So the fact that the "bones" were well preserved did
not necessarily prove that they were made of "bone."
A prominent feature of the head of every specimen is a pair of
strange rods, the occipital lamellae, projecting from the back of
the cranium alongside the vertebrae. In our new reconstruc-tion, the
shape of the articular facets showed that (as Dean had guessed) in
life they were oriented backward and downward. They are identical
with the cranial ribs found elsewhere only in lungfish. Kerr had
pointed that out in 1930, but no one had listened until Forey and
Gardiner revived his suggestions.
The next breakthrough came with the curious apparatus on the snout
that had confused so many authors, including the most recent ones. A
reasonable explanation was that it had something to do with the
nasal apparatus: either the complex nasohypophysial structure found
in lampreys and hagfish or the nasal capsules of a gnathostome.
However, I recalled that in 1879 Alexander Agassiz had described the
development of the gar Lepisosteus, a primitive teleost
that has an almost identical feature to the anterior structure in
Palaeospondylus. It is an attachment organ, a larval
adaptation lost in later life.
If the rostral apparatus is identified as a larval attachment organ,
interpretation of the rest of the head falls neatly into place. The
hemidomes are the nasal capsules and the computer images show that
Sollas's hemidome septa were their perforated dorsal walls, just as
in modern larval lungfishes. The ampyx is the paired premaxillae,
and the tauidion is the vomer. In that case, the big, chunky
gammation has to be the palato-quadrate. And the first
post-branchial plate is the dorsal part of the hyoid arch. The new
reconstructions also show that the enigmatic anterior trapezoidal
bar articulates with the front of the gammation. If the gammation is
the palato-quadrate, then the trapezial bar is possibly a rudiment
of Meckel's cartilage—the embryological basis of the lower
jaw. Suddenly, nothing present on the fossil was unaccounted for and
it was all consistent with Palaeospondylus being a larval lungfish.
» Post Comment