13
There is also paleontological evidence that xenacanthids, predominantly adapted
to freshwater, have also lived in marine environments (Hampe and Ivanov 2007 ;
Ginter et al. 2010 ).
Order Selachii (Typical Sharks)
The order Selachii and the class Chondrichthyes (sometimes also called Selachii)
include different species of sharks, which majority can be traced back to around 100
million years ago (Martin 2006 ). These carnivorous animals use of food items ranging from plankton and fi sh to marine mammals and garbage. The young are born
alive as well as hatch within the female. About 350 living species of sharks are estimated today. Some of them are small like pygmy ribbontail catfi sh shark (less than
30 cm). Others are huge in size like the whale shark with up to 12 m length. These
magnifi cent creatures are able to live in every marine niche from the Arctic to the
tropics, and play a crucial role in keeping aquatic wildlife in balance (see for review
Compagno 2001 ). These animals keep prey populations in check and also function as
apex predators eating the weakest individuals (Stelbrink et al. 2010 ).
Living sharks are divided into eight suborders (see for review http://saltwaterlife.
co.uk/ws/sharkiologist/articles/shark-evolution-and-classifi cation/ ):
1. “ Squatiniformes (Angelsharks) have been around since the Triassic period (200–
250 mya). They are comprised of 19 species. They are found mainly in mud and
sand from cool temperate continental shelves, intertidal and continental slopes,
and in deeper water in the tropics. They are identifi ed by their broad fl attened
body, short snout, large pectoral and pelvic fi ns, two dorsal fi ns towards the
end of their tail, no anal fi n and fi ve gill slits. They look similar to rays superfi -
cially. However, the gill openings are on the sides of the head, not beneath as in
rays; and the large pectoral fi ns are clearly defi ned 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) fi rst evolved during the Jurassic period (160–
200 mya). The suborder contains 9 species normally found on the continental
and insular shelves, in shallow water in temperate regions and deeper in the
tropics. They are probably the most distinct order amongst the shark groups, easily
identifi ed by their fl attened heads and long, fl at, saw-like snout (the rostrum)
complete with barbells in front of the nostrils. Lateral and ventral teeth are used
to capture and kill prey, and possibly for courtship, competition and defence. The
lateral teeth erupt as the young are developing but lie fl at along the rostrum
until after they are born. The eyes are located on the side of the head and they
have large spiracles, two dorsal fi ns and no anal fi ns. They are bottom dwelling
predators. Although data is lacking for most of the sawshark species it is known
that the Sixgill Sawshark Pliotrema warreni (the remaining eight species all have
fi ve gill slits) is ovoviviparous and produces 5–7 pups per litter.
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview
There is also paleontological evidence that xenacanthids, predominantly adapted
to freshwater, have also lived in marine environments (Hampe and Ivanov 2007 ;
Ginter et al. 2010 ).
Order Selachii (Typical Sharks)
The order Selachii and the class Chondrichthyes (sometimes also called Selachii)
include different species of sharks, which majority can be traced back to around 100
million years ago (Martin 2006 ). These carnivorous animals use of food items ranging from plankton and fi sh to marine mammals and garbage. The young are born
alive as well as hatch within the female. About 350 living species of sharks are estimated today. Some of them are small like pygmy ribbontail catfi sh shark (less than
30 cm). Others are huge in size like the whale shark with up to 12 m length. These
magnifi cent creatures are able to live in every marine niche from the Arctic to the
tropics, and play a crucial role in keeping aquatic wildlife in balance (see for review
Compagno 2001 ). These animals keep prey populations in check and also function as
apex predators eating the weakest individuals (Stelbrink et al. 2010 ).
Living sharks are divided into eight suborders (see for review http://saltwaterlife.
co.uk/ws/sharkiologist/articles/shark-evolution-and-classifi cation/ ):
1. “ Squatiniformes (Angelsharks) have been around since the Triassic period (200–
250 mya). They are comprised of 19 species. They are found mainly in mud and
sand from cool temperate continental shelves, intertidal and continental slopes,
and in deeper water in the tropics. They are identifi ed by their broad fl attened
body, short snout, large pectoral and pelvic fi ns, two dorsal fi ns towards the
end of their tail, no anal fi n and fi ve gill slits. They look similar to rays superfi -
cially. However, the gill openings are on the sides of the head, not beneath as in
rays; and the large pectoral fi ns are clearly defi ned 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) fi rst evolved during the Jurassic period (160–
200 mya). The suborder contains 9 species normally found on the continental
and insular shelves, in shallow water in temperate regions and deeper in the
tropics. They are probably the most distinct order amongst the shark groups, easily
identifi ed by their fl attened heads and long, fl at, saw-like snout (the rostrum)
complete with barbells in front of the nostrils. Lateral and ventral teeth are used
to capture and kill prey, and possibly for courtship, competition and defence. The
lateral teeth erupt as the young are developing but lie fl at along the rostrum
until after they are born. The eyes are located on the side of the head and they
have large spiracles, two dorsal fi ns and no anal fi ns. They are bottom dwelling
predators. Although data is lacking for most of the sawshark species it is known
that the Sixgill Sawshark Pliotrema warreni (the remaining eight species all have
fi ve gill slits) is ovoviviparous and produces 5–7 pups per litter.
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview