It would be a mistake to think that ancient and modern sharks are more-or-less the same. Ancient sharks differed from modern sharks in several important respects. Of course one would expect ancient and modern sharks to share certain structural features: jaws, replaceable teeth, tooth-like scales, paired fins, internal fertilization, and a cartilaginous skeleton. It is these characteristics that define them as sharks. But, even in their earliest days, these animals developed some remarkable variations on the basic sharky theme.
Cladoselache was something of an oddball among ancient sharks. A four-foot (1.2-metre) long inhabitant of late Devonian seas (about 370 million years ago), it exhibited a strange combination of ancestral and derived characteristics. Like many ancient sharks, Cladoselache had a short, rounded snout, a mouth located at the front of the head (a mouth type called "terminal"), long jaws attached to the cranium under the snout and behind the eye, cladodont teeth, and a stout spine in front of each dorsal fin. Yet it also had strong keels developed along the side of the tail stalk and a crescent-shaped tail fin, with an upper lobe about the same size as the lower (in most modern sharks, the tail is decidedly top-heavy, with the upper lobe considerably longer than the lower). In these posterior respects, Cladoselache resembles the modern mackerel sharks of the family Lamnidae, a group which includes the white shark and its close relatives, the makos and mackerel sharks. The combination of lateral keels and crescentic tail fin is highly characteristic of fast-swimming fishes such as tunas, billfishes, and mako sharks. Many paleontologists therefore believe that Cladoselache was specialized as a high-speed predator. Remarkably well-preserved specimens from the Cleveland Shale of Ohio support this notion.
Unlike most ancient and all modern sharks, Cladoselache swam the seas virtually naked. Except for small, multi-cusped scales along the edges its fins, in the mouth cavity, and around the eye, Cladoselache's skin seems to have been almost devoid of the tooth-like scales that characterize sharks as a group. Shark scales serve as more than simple armor against injury, they strengthen the skin to provide firmer attachments for swimming muscles, yet Cladoselache managed to make do almost without them. Cladoselache's fin spines were odd, too. They were unusual in being short and blade-like, composed of a porous bony material, and located some distance anterior to the origin of each dorsal fin. These fin spines may have been lighter and sturdier than the denser, more spike-like ones of other sharks. These light-weight but stout fin spines may have reduced swimming effort yet provided solid discouragement to would-be predators.
One aspect of Cladoselache's morphology is so unprecedented, it is difficult to know what to make of it. Unlike any other shark, ancient or modern, Cladoselache seems to have lacked claspers - the paired, sausage-shaped organs that male sharks use to transfer sperm into the body of female sharks. Other sharks had already developed claspers by the time of Cladoselache's appearance. The xenacanths, for example - which appeared some 50 million years before Cladoselache - had limb-like claspers supported by skeletal elements which are sometimes preserved as fossils. Diademodus, a contemporary of Cladoselache, apparently also had well-developed claspers. It seems highly unlikely that every known specimen of Cladoselache is female, so it is something of a mystery how these sharks reproduced. Yet Cladoselache obviously managed to procreate somehow, as its lineage survived for nearly 100 million years. It may seem an unpleasant idea, but perhaps Cladoselache achieved internal fertilization by partially extruding the rear part of its cloaca and using that as the organ of sperm transfer. This is the method of copulation used by most modern birds and a few modern amphibians and reptiles - namely, the caecilians (which resemble legless salamanders) and the lizard-like tuatara. We will probably never know for sure. Fossilized Cladoselache don't kiss and tell.
About the same time Cladoselache first appeared, there evolved an important group of sharks known as the ctenacanths. The ctenacanths shared numerous conservative features with Cladoselache, but also developed several more advanced ones. Like Cladoselache, the ctenacanths had cladodont teeth, jaws attached to the skull at front and back, broad-based pectoral fins, and a strong spine in front of each dorsal fin.. But unlike Cladoselache, the pectorals of ctenacanths were supported at the base by three blocks of cartilage - as in most modern sharks - allowing them greater flexibility. Ctenacanths were also different in that their fin spines were long and cylindrical, with characteristic longitudinal ridges and unique comb-like rows of tubercles (hence their name). These spines were composed of a dense enameloid material and deeply imbedded along the front margin of each dorsal fin - as in modern spiny dogfishes (family Squalidae) and bullhead sharks (Heterodontidae).
Ctenacanths are known almost entirely from abundant fossils of their distinctive fin spines (body impressions or skeletal remains of these sharks are quite rare). The best-known genus is Goodrichthyes, known from a 7.5-foot (2.3-metre) specimen from early Carboniferous deposits in what is now Scotland. Unfortunately, this specimen is contained in some 200 separate pieces of rock, and is thus rather difficult to interpret. The genus Ctenacanthus itself is represented by many species, almost all of them established on the basis of fin spines. The ctenacanths appeared in the late Devonian (about 380 million years ago - slightly earlier than Cladoselache) and persisted until the Permian, with a few hanging on into the Triassic (about 250 million years ago). But there is no doubt that their heyday - in terms of diversity and abundance - was during the Carboniferous.