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Haikouichthys
When: Early Cambrian (~530 million years ago)
Where: China
What: Haikouichthys is one of the oldest fish known. Though first described from a single specimen in 1999, it is now one of the most well understood Cambrian chordates, as a spectacular find in China unearthed over five hundred individual specimens. Haikouichthys was primarily soft bodied, and so most of these fossils are impressions. There is a limited amount of information one can glean from a squished fossil, but when you have literally hundreds of individuals of a single species, all squished in a slightly different way, most of the morphology of an animal can be determined. This ancient fish was tiny, at only about 1.2 inches (~3 cm) long on average. It had well developed anteriorly placed eyes, a notocord, and rudimentary fins along the midline of its body. It was not an armored fish, but several specimens suggest it had a cartilaginous head shield in life, a precursor to the ossified dermal armor of the later placoderms.
Haikouichthys was closely related to the ancestor of all later fish, not just the placoderms, and as fish gave rise to tetrapods, it is also one of the first representatives of our own lineage. Phylogenetic analysis place Haikouichthys near the base of all vertebrates, with some minor ambiguity as to its placement relative to the living jawless fishes, the hagfish and lamprey. These two forms are not commonly mentioned when ‘living fossils’ are highlighted, but they are the only living examples of a once diverse assemblage of jawless and mostly finless fish.
Reconstruction from Talifero via wikimedia.
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Conodonts
When: Late Cambrian to Late Triassic (500 to 200 million years ago)
Where: Globally Marine
What: Conodonts are a strange and poorly known group of animals, which existed for 300 million years. The first conodont elements were discovered in 1856, these elements are individual tooth-like structures, though their chemical composition differs from that of true teeth. They are exceedingly tiny, as seen from the image above where a number comfortably rest on the head of a pin. These elements are small, but ridiculously common in the rock record. The morphology of the structures changes though geological time. This combined with their great abundance makes them very useful for biostratigraphy. Biostratigraphy is the method of dating rock layers based upon the fossils within them. Rocks are precisely dated using radioactive decay series, but many times there are no layers which are suitable for this type of dating. Therefore, biostratigraphy allows you to date rock layers relative to one another even if you do not know their exact ages.
For over one hundred years conodont fossils were generally only used as a biostratigraphic marker, very little was known about their biology. On rare occasions elements were found in articulation, so the general position of individual structures in life was known (one of these specimens is above, with elements labeled P1, P2, S, & M) but there was no information about the animal that contained this ‘feeding basket’. Conodont elements were proposed to belong to a wide variety of taxa; several different groups of worms, mollusks, a separate phylum from anything in today’s oceans, or once even a type of plant!
In 1983 a spectacular fossil find was published - the first body fossil of a conodont. The structures were in the anterior end of a tiny animal less than 2 inches (~5 centimeters) long. Careful study of the specimen reveled a relatively large laterally placed set of eyes, evidence of a notochord, segmented structures, and tail fins. This brought a new taxonomic identity to the table for conodonts: vertebrata or at the very least chordata. Some studies place them as more closely related to us than the living jawless hagfish! After this first discovery, a few other specimens of conodont body fossils have been found. Conodonts swam though the prehistoric seas for 300 million years, feeding on microscopic organisms in the water. Their complex feeding basket composed of the conodont elements would ensure that food only flowed inward - not out.
