Are sharks primitive?

Frilled sharks (Chlamydoselachus anguineus) are among the oldest living shark species and possess many early-evolved traits. - Credit: Citron / CC-BY-SA-3.0

People often refer to elasmobranchs (sharks and rays) as ‘primitive eating machines’; an inference that seems almost entirely based on their cartilaginous skeletons.

Back in the late 1880s, German biologist Ernst Haeckel promulgated, quite incorrectly we now know, that the stages of embryonic development and differentiation corresponded to the stages of evolutionary development of the species – “ontogeny recapitulates phylogeny” as he put it. In other words, because humans (the animal that many consider to be the most highly evolved on Earth) start life with cartilaginous skeletons, which later undergo a process of ossification (turning to bone), having a cartilaginous skeleton must be a primitive trait. This is a quaint notion, but inherently wrong. Leaving aside claims that Haeckel faked his results, one major problem with this idea is that evolution has no goal. Consequently, having features similar or dissimilar to another organism is not a measure of the “advanced” or “primitive” status of an organism.

People generally seem to regard elasmobranchs as more ‘primitive’ than bony fishes. Even if we fail to consider the inherent problems associated with comparing evolutionary trends across lineages (which is a big evolutionary no no!), bony fishes and neoselachians (modern day sharks) are the products of about 425 million years of independent evolution. This makes deciphering a ‘higher’ (i.e. more advanced) form profoundly difficult. Moreover, a paper published in the journal Genetics by a French research team led by Christiane Delarbre at the Institut Pasteur back in 1998, produced a phylogenetic analysis (i.e. he used genes to infer a taxonomic scheme) confirming that the Chondrichthyes (cartilaginous fishes) are the sister group of what were the Osteichthyes (or bony fishes - now Sarcopterygii and Actinopterygii). In other words, bony fishes and cartilaginous fishes are more closely related to each other than any other group. There is also evidence to suggest that bony fishes appeared in the fossil record before the Chondrichthyes, not the other way around as is frequently cited. Indeed, according to a posting made to the discussion list SHARK-L by Aidan Martin, bony fishes, indeed bone in general, predate the first cartilaginous fishes by some 75 to 50 million years.