Assuming you all accept your status as chordates, then I'll go ahead and continue.
First, let's recap.
Biota (Living things)
Domain:
Eukarya (organisms with nucleic cells)
[informal]
subdomain Opisthokonta (eukaryotes with rear-mounted gamete flagella)
Kingdom:
Animalia (opisthokonts which ingest and digest other organisms)
Subkingdom:
Eumetazoa (animals other than sponges)
Grade:
Coelomata (Eumetazoans with an internal tubular digestive cavity)
Branch:
Bilateria (bilaterally-symmetrical tryploblast coelomates)
Superphylum:
Deuterostomia (blastopore opens anally, then orally)
Phylum: Chordata (coelomates with spinal chords)
This heirarchical system was initially devised by one Carl Linne, better known as Carolus Linneaus, and published under the title,
Systema Naturae in 1735. Linneaus was a creationist. But he still recognized that all life appeared to belong to tiers of collective groups within larger groups, within still larger groups, which he named Kingdom, Phylum, Class, Order, Family, Genus, etc. It was a good system for a while. But as taxonomy continues to become more complex than Linneaus ever imagined it could be, we find that his 18th Century taxons are bursting at the seams with new supergroups and sub-categories being discovered on several levels on every branch of the tumbleweed of life, such that we have more tiers now than we can supply names for, meaning that some of the structure now has to be revised. Anyway, moving right along...
Not all chordates have a skull. Those that do are in the subphylum, Craniata. Hagfish are craniates in that they have have a skull, but they have no jaws or spinal vertebrae to go with it. The lamprey has a skull, and vertebrae, but no jaws. Nothing has jaws without a skull or vertebrae, and that include
Chondrichthyes, (sharks and rays) where these things are there, but they're now made of cartilage instead of bone. So it appears that the skull developed first, followed by vertebrae, and then the jaw. And this is consistent with what we see in the fossil record also, where the
first fish to have skulls and backbones and all that still lacked jaws.
Since you have a skull, you are Craniate.
Since you have vertebrae too, you are also vertebrate.
Since you have jaws as well, then you are a gnathostome.
You can't be a gnathostome without being a vertebrate first,
and you can't be vertebrate unless you are already have a skull.
Just another rule evolution has to follow that creation doesn't.
The first gnathostomes were the common ancestors of cartilagenous fish and vertebrates with more calcified bones. And we have some interesting composite fossils from that era that show traits inherited by a number of subgroups at once. For instance, compare those visious bottom teeth to the rolling row system even
some ancient sharks used for their teeth.
"Psarolepis shares a number of characteristics previously believed to be unique to actinopterygians or sarcopterygians. In addition, it has several features, such as the fin spines and characters of the shoulder plate, which are associated with placoderms, chondrichthyans, or acanthodians. In overall appearance, Psarolepis most resembles a sarcopterygian, but the plates in isolation look strikingly like placoderm material."
http://palaeos.com/Vertebrates/Units/090Teleostomi/090.200.html
On the issue of cartilaginous skeletons as opposed to those made of bone, I should point out that Sarcopterygiian (and Crossopterygiian) fish had a significant percentage of cartilage in their bones. This is evident in modern coelacanths, lungfish, and the polypterus which I mentioned before. My polypterus still has much more cartilage in her skeleton than any modern (teleost) fish. So it appears that gnathostomes (jawed vertebrates) diverged at some point in the late Silurian or early Devonian period, with chondrychthyes developing progressively more cartilage in their skeletons (except in the jaw of course) while lobe-finned fish used progressively more bone except where these hinge together.
Most of the
bony fish in the Devonian period had lobed-fins, meaning fins that were attached to limbs complete with bones. But many of these, even some of the most ancient ones, also had lungs. This is evident on every surviving species including my polypterus. The development of the lung appears to immediately follow the of divergence of bony fish from sharks, and that lung was nothing more than another sort fo birth defect, an asymmetrical
distension of the buoyancy bladder common to all bony fish. Polypteriforms and other "snake-headed" fish are obligate air-breathers in their natural environment of warm, shallow, oxygen-depleted waters. And they can even use their short little limbs to amble away from drying ponds across dry land to find larger bodies of water. Other fish, even those without actual lungs, use their bouyancy bladders to suppliment their oxygen supply when the water is too warm or stagnant to respire with their gills. This is when you see fish taking "drinks" of air.
We have lots and lots more
fossil evidence than time or space permits, which consistently shows this sort of evolutionary relationship exclusively. There are very few gaps left, and they are insignificant now, because the one or two small remaining windows can't eliminate the rock-solid walls built all around them anymore. But then we have the comparative
morphologies (study of living shapes) of both extinct and extant forms, which consistently lead to
cladograms which again always indicate this same scenario. But then, as we mentioned with the analysis of cartilage-to-calcium ratios, (as well as some other chemical factors not mentioned here) with each of the surviving lines from this branch of the hypothesized tree, we see that comparative
physiology also consistently reveals this same conclusion again. And lest we forget, we also have genomic sequencing laboratories like the
Sogin lab in Massachusetts, repeatedly confirming that
DNA analysis also leads to this one conclusion, and no other. And of course periphrial fields of geology and such also confirm that this isn't just the
most likely option, its the only one that is supported by anything, and it is supported by everything. I think that means that the fundamental similarities I am describing here should be considered causal.
Cladists (people who really get into this classification system) tend not to use the word, "fish". Somehow, that word seems to have no meaning for them. They say "vertebrates" instead. Similarly, Sarcopterygii doesn't just refer some ancient order of fish which are now mostly extinct. It refers to them and everything scientists believe descended from them.
Basically,
sarcopterygii includes all organic RNA/DNA protein-based, metabolic, metazoic, nucleic, diploid, bilaterally-symmetrical, digestive, tryploblast, opisthokont, deuterostome coelemates with spinal chords, skulls, backbones, jaws, lungs, and legs. Do any of you have reason to oppose you're being assigned to this class?