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A Palaeontological Puzzle Solved?

Keith Thomson

Head Case

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." 

Figure 2. Digital reconstructionClick to Enlarge Image

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.

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