A Level Secondary Science

When fish wore armour

Armour was all the fashion of the day in the Silurian and Devonian periods and is particularly associated with a group of heavily armoured fish, the Placoderms. This group of highly successful animals lived from early Silurian through to the end of the Devonian (443.8 – 358.9 Ma ago).

In comparison with animals met in previous articles, we are now describing animals with unmistakable vertebrate features: the dorsal nerve cord is surrounded by a calcareous structure forming a vertebral column, the first 3 gill arches have been used by that jobbing builder, Mr Evolution, as a structure to support a jaw complete with hard apatite (real name of material, how appropriate!) containing teeth.  Externally, paired fins develop and through time there are internal structures to support them: bony rays in the fins and girdles of bone to attach the fins to the vertebral column.  It is interesting that these still primitive animals show evidence of internal fertilisation and live birth. Placoderm males had pelvic fins (that’s the ones at the rear of the animal, not visible in the photo below) which were used to transfer sperm into the females and fossils have been found of Placoderm females who apparently died at the time of giving birth.

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