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PALAEOZOIC FISH UK
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Vertebrate origins and the first fish

Dickinsonia costata

Dickinsonia is the iconic Ediacaran fossil, a characteristic late Precambrian megafossil of Australia and the Russian White Sea. Considered to be have an annelid grade of organisation it has also been compared with jellyfish, scleractinian corals, or extinct Vendobionta. . It possesses cephalization (a head end), segmentation and a through gut can be clearly seen running through the center of the specimens.

Pikaia from the Mid-Cambrian Burgess shale site of Canada is the first animal known to have a notochord and a tail fin supported by rods of cartilage. Pikaia is now considered to be a cephalochordate or Pre-craniate.

Pikaia

Haikouella lanceolata, thought by its describers to be the earliest craniate-like chordate is found in the Chengjiang formation. This fish-like animal has a discernible heart, dorsal and ventral aorta, gill filaments, and a neural chord( Nature 402, 518-522, 02)

Haikouella lanceolata

Haikouella, Lower Cambrian (525MM), Quiongzhusi Section, Yu’anshan Member, Heilinpu Formation, Anning, Cheingjiang County, Yunnan Province, China.

Vetulicolians, common in the Lower Cambrian deposits, were first classified as arthropods because their body consists of a large shield-like carapace and a segmented tail. However, upon more study it became apparent that the arthropod resemblance was superficial and as well as the body being composed of two distinct sections, they have a central row of markings that look like simple gill slits. Interestingly, many years ago the famous paleontologist Romer suggested that the ancestor of the fish may be a strange animal, constructed of an anterior with gill slits, and a elongated, segmented posterior!!

Vetulicola gantoucunensis, Lower Cambrian Wulongqing Formation in the Kunming

Animal showing gut getail

Vetulicola rectangulata

Quite how evolution got from the invertebrates to these first chordates is a subject of continual debate. Candidates include the stylophoran carpoid echinoderms, tunicates (sea squirts) and even the concodont animal. At one stage even the arthropod Ainiktozoon was considered a possible chordate, despite its compound eyes.

Ainiktozoon loganense Scourfield. Silurian (Llandovery) of Lesmahagow

Simplified drawing based on the work of D..J. Scourfield (1937) and A. Ritchie (1985) both of whom interpreted Ainiktozoon as a proto-chordate

Most paleontologists now reject the calcichordates as the ancestors of the Craniata and some favor the Conodont animal

The first fish appear at the beginning of the Ordovician about 470 Myn years ago. Probably the best known are Sacabambaspis from Bolivia and Arandaspis from Australia. These jawless fish have a bony fusiform head and a tail covered with rod-like scales. These fish had no paired fins and only had a caudal fin. These Arandaspida are thought to have lived in the shallow coastal waters, surviving by scooping algae-rich mud from the sea floor.

Sacabambaspis

Harding Sandstone, Colorado, USA. Ordovician (M. Caradoc age)

Charles Walcott first described the vertebrate fauna of the Harding Sandstone in 1892 and the fauna is now recognized as one of the richest Ordovician vertebrate localities in the world. The fauna is particularly important because it shows that vertebrates began their evolutionary radiation in the early Palaeozoic. The majority of vertebrate fossils from the Harding Sandstone are isolated dermal plates and scales. The only teeth are those that belong to the conodonts