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ORDOVICIAN PERIOD

488-444 million years ago

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Late in the Cambrian, early vertebrates evolved bone although it was distinct from that of modern vertebrates.

Early jawless and jawed fish developed dermal bone as head armor, forming the third portion of the skull, the dermatocranium. Dermal bone in the developing human embryo will form the following bones in humans: premaxilla, maxilla, nasal, lacrimal, zygomatic, squamosal (part of temporal), frontal, parietal, vomer, palatine, pterygoid, paraspheniod (the previous 2 forming parts of the sphenoid), and dentary. Placental mammals have lost many of the dermal bones present in more primitive vertebrates such as the prefrontal, postfrontal, postorbital, intertemporal, supratemporal, tabular, quadratojugal, postparietal, ectopterygoid, splenials, angular, surangular, and a number of bones present in fish (such as the opercular series).

Fossil jawless fish (such as the ostracoderms) and the primitive gnathostomes (placoderms) possess a dermatocranium composed of large plates of bone. In the most primitive bony fish (acanthodians, pictured below) the dermatocranium is composed of many smaller bones whose patterns resembles that of higher fish although establishing precise homology is difficult (Carroll, p. 86).

The first vertebrate bone fragments are known from the Cambrian. These fragments are only a few square millimeters in size and differ from the bone of modern vertebrates. The fragments have been identified as belonging to a jawless fish, named Anatolepsis. Anatolepis was probably only lightly armored and inhabited continental shelves at about the same time as the conodonts did (Smith, 1996). The first complete specimen of a jawless fish with bone, Arandaspis, is known from the Ordovician.

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