The Evolution of SharksSimon Vetterli
Shark fossils have helped scientists conclude numerous facts about the evolution of sharks. They are believed to have first appeared on the planet nearly 455 million years ago.Doliodus Problematicus is the name given to the fossil thought to be the oldest discovered thus far. It is an acanthodian (ak-uhn-thoh-dee-uhn), as well as a chondrichthyan. A large word to mostly describe a skeleton of cartilage, which typically corrodes leaving no clues for future enthusiasts; fortunately for us, Doliodus Problematicus’ fossilized, giving us a glimpse into the past and a nice peak into the evolution of sharks.
The Shark Population
The various types of sharks were at a high c. 360 mya, but by the ending of this period (also known as Carboniferous) and the beginning of the Permian age, many of them had become extinct. As nature tends to resolve its own problems, a profusion of ray-finned fish populated the oceans and since they are a preeminent food source, sharks once again flourished as a diversified species and the evolution of sharks continued.
Some Features Haven’t Evolved
Fossils from the Triassic age of the evolution of sharks revealed the Palaeospinax shark, which is mostly related to today’s Dogfish species. They share similarities of jaw placement, dorsal fin spines and a vertebral column that is sectioned. Squalidae is another name for the Dogfish family whose most famous members are the Spiny dogfish and the Spurdog, aka Squalus acanthias, which have maintained the dorsal fin spines of their ancestors.
To date, sharks have managed to keep the majority of their distinct physical characteristics. The most notably being the torpedo shaped shark, but not to be underestimated are the various other conspicuous qualities including the sizable filter feeding sharks, those with a flattened bottom, the deep water slow moving ones and of course camouflaged sharks.
The Great Megalodon Shark
One of the most famous ancient sharks is the Megalodon shark. The Megalodon shark (Carcharodon Megalodon) was a massive being that would put modern day sea predators to shame. If you are currently afraid of the great white shark, then I’m sure you’re happy that you don’t have to worry about the megalodon shark creeping up under your sailboat. This colossal being was not one to mess with, and although the megalodon is now extinct, its memory most definitely deserves to be respected and well known.
It is the varying forms of these magnificent creatures that help determine their classification throughout the evolution of sharks. Thus far they have been divided into eight categories:
Angel Sharks (Squantiniformes), have 19 variations, they enjoy the deep water tropics and are also traditionally found on mud and sand. They reportedly swim in cool temperate shelves, continental slopes and inter tidal waters. Needless to say, you might have the pleasure of seeing them while on vacation.
Similarities to Rays
Distinguished by their sizeable pelvic and pectoral fins, Angel Sharks lack an anal fin but have 2 dorsal fins near the end of their tails and 5 gills. Rays share a comparable resemblance, but the gill openings on a ray are beneath it and the Angel Sharks’ are on the sides of their heads. Angel Sharks lay covered in the sand and easily ensnare prey with a sudden attack from ample mouths that are situated precisely in front of their heads.
Offspring and reproduction
With an interesting reproductive capability, their eggs hatch within them, making themOvoviviparous; their offspring are born alive in litters ranging from 1 to 25.
Saw Sharks (Pristiophoriformes), date back to Jurassic times within the evolution of sharks, 160-200 mya., and have 9 known species. If you search to get a glimpse at these beauties, they hang out in the deeper tropics, on the continental in the shallow waters of temperate regions and insular shelves (naturally occurring, within 600 feet, underwater sea beds that exist beyond the boarder of an island that lacks sleep mountains (mountains that thrust directly into the sea).
They are famous for flattened heads and rostrums (snouts), which resemble a flat, long saw (hence their nickname). Their ventral and lateral teeth may or may not assist them when courting, for defense and competitiveness, but they are absolutely employed in the securing and slaughter of their prey!
Offspring And Reproduction
They too are Ovoviviparous like the Angel Shark, but instead of litters up to 25, Saw Sharks only have 5 to 7 pups. In the development stage their teeth erupt, but manage to remain against their rostrums in a flat position until they are born. They are true predators with 2 large Spiracles (openings behind their eyes that receive oxygenated water and sustain their ability to breathe on the ocean floor and underneath the sand). Their eyes are on the sides of their heads and though they lack anal fins, they have 2 dorsal ones.
Unfortunately like most species of sharks, Saw Sharks do not have nearly the amount of information we shark fans crave, but I can add that only one of its species has 6 gills! Known as the ‘Six Gill’ Saw Shark (Pliotrema warreni), and all of the other 8 of their species have 5 gill slits.
Dogfish (Squaliformes) were initially found in 160-200 mya, the Jurassic age. They are known throughout time and have been recognized by specialists.
Offspring And Reproduction
Although they can typically have a litter containing only 1 pup, they still manage to have seven families with 106 noted species. The families that these 106 are divided into are:
Kitefin sharks (Dalatiidae) – Bonnaterre described them as Squalus licha in 1788, which was later changed to Dalatias lichia. Although they experience several scientific titles over the years, common names for them in the English language include darkie charlie, seal and black shark. Being somewhat popular, they have been given varying names in other languages throughout the world. So many in fact that I have chosen to spare you the long list, instead, I’ll continue with the 7 families and recommend you stay tuned into the sight and review my Types of Sharks article to become more knowledgeable about your favorites or possible gain a new fav!
The remaining Sawshark families are:
- Bramble sharks (Echinorhinidae)
- Dogfish sharks (Squalidae)
- Gulper sharks (Centrophoridae)
- Lantern Sharks (Etmopteriade)
- Sleeper sharks (Somniosidae)
- Roughsharks (Oxynotidae)
Frilled and Cow sharks
Frilled and Cow sharks (Hexanchiformes) are an ancient living shark from 260-300 mya (Permian period) and are seen in the evolution of sharks. Their 2 families are Frilled sharks (Chlamydoselachidae), which are compared to eels having distinguishingly spaced teeth, and Cow Sharks. Frilled sharks have 4 species of their own and like the Cow sharks (Hexanchidae), they have 6 to 7 gill slits, a single dorsal fin and anal fins. Cow sharks have 2 species of their own and are shaped cylindrically. Located throughout the worldwide, they are predominantly found in tropic areas in deep cold water. Being Ovoviviparous, frilled sharks have 6 to 12 pups per litter and cow sharks anywhere from 6 to 108 pups! Can you say babysitters on speed dial?
The most widespread and largest order of sharks date back to 160-200 mya, the Jurassic period. They are known as Ground Sharks (Carcharhiniformes) and have 247 species within their 8 families. They are located in various conditions, intertidal to deep waters, pelagic open ocean, and from cold to tropical seas. Although the family resemblance significantly varies, the common features are anal fins, 5 gill slits and other than the Onefin catshark (Pentanchus profundicolus) they all have 2 dorsal fins.
Lamniformes (Mackerel sharks) date from 160-200 mya (the Jurassic Period). They are found in the open ocean, intertidal zone and deeper water. There are 15 species in 7 families.
A bit of intriguing information about one family member the Sandtiger shark (Carcharias taurus) has cannibalism amongst its pups – the dominant will consume its sibling embryos.
It is ironic that someone in the distance is vacuuming as I write about the Carpet Shark(Orectolobiformes) from 160-200 mya, (the Jurassic period). They have 33 species within 7 families!!! They all swim in warm waters, tropical seas and intertidal zone to deep waters. Other than the Whale shark (Rhincodon typus) all family members are bottom dwellers.
Last but I’d never call it least to its face, is the Bullhead shark (Heterodontiformes). Dating back to 200-260 mya (the Triassic period) they are stout-bodied bottom dwelling, with 2 spined dorsal fins and anal fins. With 9 species, their habitat varies from one species to another, the intertidal zone, continental and insular shelves, and are nocturnal(remember that sharks don’t sleep!!!) they prefer resting in caves and rocky crevices during the day. Although the number of eggs they lay is still unknown, their eggs are distinctively screw shaped.
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