Fossil Review Part 2 - Devonian Period


This is the second in our series of blogs uncovering the fossil collection at Paisley Museum and highlighting some of the weird and wonderful creatures that have played their part in the evolution of life on Earth.

Devonian Period (419 – 359 million years ago)

Often called the “Age of the Fishes” the Devonian period was characterised by a rapid diversification in fish.

The earliest fish in the fossil record date from the 542 – 485 million years ago. These were jawless fish, many of which were armoured with heavy, bony plates. The only two surviving groups of jawless fish are the lampreys and the hagfish.

Jawed fish first evolved in the Silurian (443 – 419 million years ago) but it was during the Devonian that this group dominated and that the diversification of one group of the lobe-finned fish led to the evolution of the first tetrapods (four-limbed, land vertebrates).

Pterichthyodes oblongus from the Lower Old Red Sandstone of Cromarty, Highlands, Scotland

One of the placodermi, an extinct group of armoured, jawed fish, that lived from the Silurian to the end of the Devonian. Their head and thorax were covered by jointed, armoured plates.

Placodermi were a highly successful and diverse group and appear to represent an “early experiment” in the evolution of jawed fish, adapting into a number of environmental niches (Pterichthyodes oblongus was a bottom-dweller) and body shapes, before becoming extinct.

This specimen was collected by Hugh Miller (1802 – 1856), a pioneering, but almost entirely self-taught Scottish geologist whose discoveries on Devonian fossil fish saw him rise to become one of the pre-eminent geologists of his time.

Coccosteus decipiens from the Middle Old Red Sandstone of Edderton, Highlands, Scotland

 

Another specimen from the placodermi group of armoured, jawed fish, but this species belonged to a sub-group, the arthrodira (Greek for “jointed neck”) which had a flexible joint between the armour body plating of the head and chest. Additionally Coccosteus had an internal joint between the neck vertebrae and the back of the skull allowing its jaws to open wider and so to feed on larger prey.

Osteolepis panderi from Hill of Forss, Thurso, Highlands, Scotland

This species belongs to the sarcopterygii or lobe-finned fish of which there are only two living members, coelacanths and lungfish. The largest of the lobe-finned fish was Rhizodus hibberti from the Carboniferous period which we will meet in a later blog.

Lobe-finned fish are characterised by paired pectoral and pelvic fins joined to the body by a single bone. These paired fins gave rise to the paired limbs of tetrapods with the single bones evolving into the humerus (fore-limb) and femur (hind-limb).

Holoptychius flemingi from the Upper Old Red Sandstone of Dura Den, Cupar, Fife, Scotland

Another member of the sarcopterygii or lobed-finned fish, this species lived in the freshwater lochs of an arid, desert environment (which was the Scottish climate during the Devonian period).

Found at the famous Dura Den site where most of the fossils are crowded into one rock bed which marks the mass-death of fish trapped as the waters dried up during a period of drought. These fossils show the large, bony scales preserved as the body of the fish disintegrated.

 

 Equally significant to the diversification of fish during this period was the diversification of plants. Plants began to invade the land during the Ordovician (485 – 443 million years ago), but it was during the Devonian that they began to spread inwards from the coastlines, establishing new land ecosystems in the form of the first rainforests.

Archaeopteris hibernica from the Upper Old Red Sandstone of Kiltorcan, Ballyhale, County Kilkenny, Ireland

 

From the Greek archaios meaning “ancient” and pteris meaning “fern” and not to be confused with Archaeopteryx the “first bird”, the name of which actually means “ancient feather “ or “ancient wing”.

Archaeopteris was essentially the first modern tree. Rather than growing just from its tips, Archaeopteris evolved a woody tissue which allowed it to undergo a secondary growth increasing the girth of its roots and stems, thus providing support for taller growth.  

However, unlike modern trees Archaeopteris produced spores (just like modern-day ferns) rather than seeds.

 

 

Two major coral groups emerged during the Ordovician (485 – 443 million years ago), the tabulate and the rugose corals and continued to evolve throughout the Silurian (443 – 419 million years ago), peaking in diversity during the Devonian. The diversification of these two coral groups provided important habitat for other invertebrates.

Not related to modern corals, these two groups became extinct at the end of the Permian period 252 million years ago.

Thamnopora cervicornis from Devon, England

 

Thamnopora was a tabulate coral and as such a colonial coral. As in modern reefs the coral type and shape is dependent upon the environment in which they grow. Thamnopora was a branched coral growing in bushes and, as with modern branching corals, was most probably found in strong water current areas.

Heliophyllum halli from Bethany, Genesse County, New York, U.S.A

 

Both solitary and colonial rugose (meaning “wrinkled” or “rough”) corals are known, but the solitary types, also known as “horn corals” due to their shape, are more common.

Heliophyllum was a solitary coral, although the photograph shows a second coral developing on the rim. Like modern corals, these ancient types most likely fed in the same way using tentacles to capture prey.

 

By Nicola Macintyre at Paisley Museum 


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