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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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