Lately I’ve been curious about the phylogenetic relationship of the batoids (skates and rays) with respect to the sharks. One source (Compagno 2005) describes the batoids as “flattened sharks” and depicts cladograms showing the batoids as the sister group to the Pristiophoriformes (sawsharks), while I’ve seen another reference showing batoids as one of a trichotomy with the squalids and galeids. Does anyone know what the most current thinking on this issue is? Are skates and rays, phylogenetically speaking, sharks? Does that mean that sharks, as traditionally defined, are a paraphyletic grouping?
I do not have anything at all current on Chondrichthyes generally, much less at the level of detail you’re looking at, but I’m quite confident that unless some really remarkable findings have been made in the last ten years, the skates, rays, sawfish, guitarfish, and related forms are all specializations from the “modern shark” basal line, meaning that the answers to your last two questions are both “Yes.” (And this is based purely on my personal updating of my own knowledge of phylogenetics a year or two ago with the aid of a 1996 reference, and paying attention to modern sharks.)
Hopefully someone with the proper background will be along to do a more accurate and detailed answer.
Thanks for the response!
A little more digging turns up a reference to de Carvalho (1996), who also places batoids deep in the Squalea as the sister group to the pristiophorids; (Squatiniformes + (Pristiophoriformes + Batoidea)) forming the clade Hypnosqualea, which is the sister group to the Squaliformes.
So I think that unless someone has references to the contrary, I’ll accept that as the prevailing idea.
Didn’t the bony fishes have a cartiliginous ancestor? Was that ancestor (assuming I’m correctly remembering its existence) in the shark clade, as well?
Yes and no, in two ways.
First, there seems to be fairly good evidence that cartilage preceded bone in the earliest stages of Craniate (=Vertebrate) evolution. However, that has very little to do with the shark/bony-fish dichotomy, as it precedes the phylogenetic differentiation of “fish” classes (in the Linnaean sense). Bone as a structural unit in vertebrates preceded Selachii; sharks simply make little if any use of bone for skeletal purposes.
Second, what you’re thinking of is most likely the “chondrosteans,” about which more in a minute. And this is the point at which aquatic-vertebrate phylogeny needs to be sorted.
There are at least five extinct and two living groups of jawless “fish” – the five dead groups traditionally lumped together as the Ostracoderms, plus the lampreys and hagfish. All seven groups are as cladistically distinct from each other as they are from any jawed vertebrate, from shark to guppy to hyena.
There are at least four main groups of jawed aquatic vertebrate – “fish” in the general sense – two living (Chondrichthyes and Osteichthyes) and two extinct (Acanthodii and Placodermi). In addition, you will get all sorts of arguments about the proper classification of the chimeras, the small large-headed deep-water fish that are traditionally considered the closest relative of the Selachii, whether the various groups assigned to the Placodermi are in fact parts of a single or multiple clades, and various other issues to make a polemic taxonomist wax verbose.
The two living groups both seem to have sprung from an ancestor within or close to the Acanthodii (the “spiny sharks” that were a minor but consistent constituent of the fauna from the Silurian well into the Permian). They are probably taxonomically distinct – i.e., there is no common ancestry of Chondrichthyes and Osteichthyes/Tetrapoda that does not also include other groups.
The Chondrichthyes may or may not include the Chimeras, and the sharks sensu lato, for the details of whose phylogenetic history I gladly yield to Pazu. As noted, the sawfish, sawsharks, skates, rays, and other groups fall into this. Interestingly, there are Paleozoic deposits with what are definitely early selachians but occupying typical modern small-fish roles: gobies, wrasses, and the like.
The Osteichthyes immediately fall into two major subgroups: the Sarcopterygii and the Actinopterygii – fleshy-finned and ray-finned fish. While there were scads of sarcopterygians in the Devonian, filling a wide assortment of econiches, today’s survivors include only three forms of lungfish, the coelacanths, and the tetrapods.
The vast majority of bony fish are actinopterygians, with fins of thin tissue over rays, rather than flesh on bone as in the sarcopterygians and as tetrapod limbs. But these went through three episodes of radiation and divergence, traditionally termed the Chondrostean, the Holostean, and the Teleostean. The first two are characterized by ganoid scales, other scale patterns developing among the teleosts. And of course the first two are paraphyletic, only the teleosts comprising a clade (if even they do).
However, the nomenclature is misleading. All three radiations were characterized by predominatly bony fish, presumably with some cartilage here and there. However, the principal survivors of the Chondrostean radiation (the sturgeons and bichirs) have in common that they have lost the majority of their skeletal bone, using a cartiliginous structure instead – though they retain dermal bone! (The other surviving Chondrostean, if it is one, is the paddlefish Polypterus, sufficiently divergent from everything else to make it very difficult to classify; some taxonomists consider it a Sarcopterygian.)
Owing to the fact that modern Chondrosteans do not have much skeletal bone, the idea of an ossified skeleton was for some time thought to be a Holostean development. However, the cartiliginous skeletal structure of modern chondrosteans is a secondary development, an adaptation for their lifestyle and not a primitive character.
So, to be sure I’m understanding that correctly, let me summarize: Some critter evolved a cartelige skeleton. Some of that critter’s descendants eventually hardened the skeleton into bones, and developed jaws. Of those critters with the bones and jaws, some descendants (including carp, trout, and us) kept them, and other critters (the shark clade) kept the jaws, but went back to cartilige skeletons instead of bones. Is that correct?
Pretty much. Note that the bone–> cartilage phenomenon happened at least twice, in far different creatures: the shark/ray clade, and the sturgeon/bichir clade. And that both retain dermal bone – small particles of bone material embedded in the scales for protection.
Wonderful summary, Polycarp!
A few points:
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According to at least a couple of recent references, chimaeras are indeed placed within the Chondrichthyes, in the Holocephali (sister group to the Elasmobranchii, which comprises (Cladoselachimorpha + Euselachii)).
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The paddlefish is Polyodon, not Polypterus. The latter genus is the bichir, which you mentioned previously. My understanding was that the paddlefish was the sister group to the acipenserids (sturgeons), together comprising the Chondrostei, while the bichir was considered a basal actinopterygian and the sister clade to all other extant actinopterygians. But I could be completely out of date on this.