Shark Evolution and Classification
True sharks in the class chondrichthyes evolved approximately 455 million years ago, in the late Ordovician period. They had cartilaginous skeletons, their skin was covered in tooth-like denticles and their jaws were lined with rows of teeth. We know this due to various types of fossil evidence such as teeth, fin spines and rare skeletons. The oldest shark fossil found to date named Doliodus problematicus dates back to the Early Devonian period almost 409 million years ago. It has been identified as an acanthodian and a chondrichthyan. Fossils like this are rare because cartilage usually decays quickly leaving little time for fossilisation to occur.
During the early Carboniferous period (c. 360 mya) shark forms diversified greatly, with species evolving and adapting to their environment and their food source. This was followed by a decrease in diversity with many forms becoming extinct by the beginning of the Permian period (c. 300 mya). However by the end of the Permian (c. 250 mya), there is evidence to suggest an abundance of ray finned fishes (Actinopterygians) inhabited the oceans, these Actinopterygians were an excellent source of food for the early shark forms and in response sharks began to diversify once more.
The fossil record for the early Triassic period (c.250 mya) shows a species known as Palaeospinax, this shared a similar morphology (dorsal fin spines, a sectioned vertebral column, position of jaw) to shark species found today such as the dogfish family (Squalidae), most notably the Spiny dogfish or Spurdog (Squalus acanthias) which has retained the dorsal fin spines.
Present day shark species have retained their diversity and variation, this can be seen by taking a look at their physical appearance; from flattened bottom dwelling sharks, to large filter feeding sharks, slow moving deep water sharks, camouflaged sharks, eel like sharks and of course the more familiar sleek torpedo shaped sharks.
Different body shapes and features are amongst the characteristics used to classify the eight separate orders of sharks which are as follows:-
1. Squatiniformes (Angelsharks)
…have been around since the Triassic period (200-250 mya) they are comprised of 19 species and are found mainly on mud and sand from cool temperate continental shelves to intertidal and continental slopes and also deeper water in the tropics. They are identified by their broad flattened body, short snout, large pectoral and pelvic fins, 2 dorsal fins towards the end of their tail, no anal fin and 5 gill slits. They look similar to rays however the gill openings are on the sides of the head not beneath as in rays and the large pectoral fins are clearly defined and separate from their heads. They have large wide mouths at the front of their head perfectly designed for ambushing their prey as they swim by the often sand covered shark. They are ovoviviparous (they produce live young from eggs which hatch within the body) with litter sizes between 1 and 25.
2. Pristiophoriformes (Sawsharks)
… first evolved during the Jurassic period (160-200 mya). Comprised of 9 species these are found on the continental and insular shelves, in shallow water in temperate regions and deeper in the tropics. Probably the most distinct order amongst the shark groups easily identified by their flattened heads and long, flat, saw-like snout (the rostrum) complete with barbells in front of the nostrils and lateral and ventral teeth which are used to capture and kill prey as well as possibly for courtship, competition and defence. The lateral teeth erupt as the young are developing but lie flat along the rostrum until after they are born. The eyes are located on the side of the head and they have large spiracles, 2 dorsal fins and no anal fins. They are bottom dwelling predators. Although data is deficient for most of the sawshark species it is known that the Sixgill Sawshark Pliotrema warreni (the remaining 8 species all have 5 gill slits) is ovoviviparous and produces 5-7 pups per litter.
3. Squaliformes (Dogfish)
…have been around since the Jurassic period (160-200 mya). This is a large and varied order containing 106 identified species in seven families; bramble sharks (Echinorhinidae), dogfish sharks (Squalidae), gulper sharks (Centrophoridae), lantern sharks, (Etmopteridae), sleeper sharks (Somniosidae), roughsharks (Oxynotidae), and kitefin sharks (Dalatiidae).
The dogfish sharks habitat is wide ranging with species found in marine estuarine environments world-wide. Currently they are the only known sharks to be found at high latitudes close to the poles. Their greatest diversity occurs in deepwater. They have a cylindrical/torpedo shaped body with the eyes on the side of the head, 5 gill slits, 2 dorsal fins (in some species these are spined), and no anal fins. They are ovoviviparous with some species having a low fertility of just 1 pup per litter (e.g.: the gulper shark Centrophorus granulosus) and other species having a very long gestation period of 18-24 months (e.g.: the spiny dogfish Squalus acanthias).
Some species are thought to be solitary; others form schools that range long distances during seasonal and annual migrations.
4. Hexanchiformes (Frilled and Cow sharks)
…are considered to be amongst the most “ancient” forms of living sharks dating back to the Permian period (260-300 mya). They are comprised of 2 families; cow sharks (Hexanchidae of which there are 2 species) and frilled sharks (Chlamydoselachidae of which there are 4 species). The frilled sharks are eel-like with distinctive spaced out teeth, the cow sharks are the more conventional cylindrical shape. Both families have 6 or seven gill slits, 1 dorsal fin and anal fins are present. Most of the species are found worldwide, predominantly in deep cold water of the tropics. They are ovoviviparous with frilled sharks producing 6-12 pups per litter and cow sharks ranging from 6-108 pups depending on the species.
5. Carcharhiniformes (Ground Sharks)
…are the largest, most diverse and widespread order of sharks. Dating from the Jurassic period (160-200 mya) the ground sharks are comprised of around 247 species in 8 families; catsharks (Scyliorhinidae), finback catsharks (Proscylliidae), false catsharks (Pseudotriakidae), barbeled houndsharks (Leptochariidae), houndsharks (Triakidae), weasel sharks (Hemigaleidae), requiem sharks (Carcharhinidae), and hammerhead sharks (Sphyrnidae).
These shark species inhabit cold to tropical seas, intertidal to deep water and pelagic open ocean. Their physical appearances can be quite different; from the Daggernose shark (Isogomphodon oxyrhynchus) to the Great Hammerhead (Sphyrna mokarran), however all have 5 gill slits, 2 dorsal fins (with one exception; the aptly named “Onefin catshark” Pentanchus profundicolus) and anal fins.
Reproductive strategies are also very varied both the catsharks and the finback catsharks are oviparous (egg laying) and ovoviviparous with typically 1-2 eggs or live pups per litter, false catsharks are ovoviviparous (2-4 pups per litter), oophagous (meaning “egg eating” pups feed off eggs produced by the ovary whilst inside the uterus, 2 pups per litter), viviparous (pups nurtured via a placental connection, 7 pups per litter), the barbeled houndsharks reproductive method is unknown, houndsharks are ovoviviparous (2-52 pups per litter) and viviparous (2-20 pups per litter), the exact method in weasel sharks is unknown but they do bare 1-4 live young, requiem sharks are ovoviviparous (10-80 pups per litter) and viviparous (1-135 pups per litter) and hammerhead sharks are viviparous (30-55 pups per litter).
6. Lamniformes (Mackerel sharks)
…date from the Jurassic Period (160-200 mya), they consist of 15 species in 7 families; thresher sharks (Alopiidae), Cetorhinidae, Lamnidae, Megachasmidae, Mitsukurinidae, Odontaspididae, Pseudocarchariidae (no common names exist for the latter 6 families).
These species are predominantly large active pelagic sharks with cylindrical/torpedo shaped bodies with 5 gill slits, 2 dorsal fins and anal fins. They are found worldwide from the intertidal zone to deeper water and the open ocean. Reproduction is ovoviviparous (2-25 pups per litter depending on species) with 1 species the sandtiger shark (Carcharias taurus) cannibalism occurs as the dominant pup will consume the other embryos.
7. Orectolobiformes (Carpet sharks)
…originating from the Jurassic period (160-200 mya), this order consists of 33 species in 7 families; collared carpetsharks (Parascyllidae), blind sharks (Brachaeluridae), wobbegongs (Orectolobidae), longtailed carpetsharks (Hemiscyllidae), nurse sharks (Ginglymostomatidae), zebra shark (Stegostomatidae), whale shark (Rhincodontidae). They can be found worldwide in warm temperate and tropical seas, from the intertidal zone to deep water. With the exception of the whale shark (Rhincodon typus) all are bottom dwelling, they have 5 gill slits, 2 dorsal fins and anal fins. They utilise a variety of reproductive strategies including oviparity (6-8 egg cases), ovoviviparous (20-30 pups per litter), viviparous (up to 300 pups per litter) and oophagy (litter size unknown).
8. Heterodontiformes (Bullhead sharks)
…date back to the Triassic period (200-260 mya). Comprised of 9 species they are bottom dwelling stout-bodied sharks, with 2 spined dorsal fins and anal fins. Their habitat varies between species from the intertidal zone to continental and insular shelves, they are nocturnal preferring to rest in rocky crevices and caves during daylight. Reproduction is oviparous, laying very distinctive egg cases which are screw shaped; the number of eggs laid is unknown.