Fossil Friday: The Time When Fish Ruled the Earth
The Silurian Period could be called the time when fished ruled the Earth. Fish rapidly diversified throughout the period. There are two main groupings of fish that evolved: jawless and jawed.
Jawless fish first appeared in the Late Ordovician: Astraspida and Arandaspida. Astraspids are small, armored jawless fishes characterized by a dermal ornamentation of large, star-shaped tubercles of fine-tubuled dentine ("astraspidine"), covered with a thick, glassy cap of enameloid. They are represented by a single genus Astraspis.
Arandaspids are another small, armored fish with a flat rather than oval-shaped dorsal shield. These are represented by several genera of which Sacabambaspis is the most well-known. Check out my post on Sacabambaspis.
In the Silurian, several new groups of jawless fish appeared including thelodonts, heterostracans, osteostracans, and anaspids. Thelodonts were some of the first fish with scales rather than plates of armor. They first appeared in the Ordovician but they flourished in the Silurian. They lacked an ossified skeleton and the scales were either poorly attached or not attached to each other at all which makes finding a complete one incredibly difficult. The scales were small, 0.5-1.5mm, and didn’t overlap. They were teardrop-shaped and pointed on one end giving them there name “nipple tooth” They were ridiculously common in their habitats and globally distributed making them useful biostratigraphic markers.
Heterostracans also appeared in the Ordovician and radiated rapidly in the Silurian. Consisting of around 300 species, their head shields differed greatly from one another. Most had two plates which formed the dorsal shield and one large ventral shield with a series of scales arranged along the sides. The scales are composed of three layers of dentine and aspidine (an acellular bony tissue unique to them and thelodonts). The middle layer was honeycombed with tiny holes called cancella. One distinguishing characteristic was their single exhalant opening into which the gills open.
Anaspids were small armor less jawless fish. In fact, the name means “shield less ones”. They are covered in small, weakly mineralized scales and a row of large scutes running down the back. A major synapomorphy is the large tri-radiate spine behind the gill openings.
Osteostracans were the first group of fish with paired fins. They are often called ostracoderms, they were similar to lampreys in possessing two pairs of semicircular canals in the inner ear (as opposed to three in jawed vertebrates). These fish are the sister group to jawed fishes.
The jawed fishes first appeared in the Silurian Period. Four major groups radiated during this time: placoderms, acanthodians, cartilaginous fishes and bony fishes. Placoderms were mostly predatory armored fishes similar to the jawless ones. They were some of he earliest jawed fish and were the first ones to develop pelvic fins, the second set of paired fins. Some species have been found to be viviparous.
Acanthodians were small fishes with slender bodies. Th paired fins were modified to long spines earning then the nickname “spiny sharks”. Unlike most sharks, acanthodians lacked teeth. Many even moved to freshwater.
Chondrichthyes or cartilaginous fishesmay have evolved from acanthodians. This group consists of modern sharks, rays, skates and chimaeras.
Finally, the first Osteichthyes or bony fishes appeared in the Silurian Period. Unlike all the other fish, they have an endoskeleton made of bone tissue.
Thanks for coming to today’s lesson and tune in Monday to learn about the climate of the Silurian Period. Have a good weekend and fossilize you later!
