Incisoscutum is an extinct genus of placoderm. Well preserved fossil embryos in the body cavity of Incisoscutum showed that these fishes, close to the common origin of all jawed vertebrates, gave birth to live young in a manner similar to modern sharks

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

. Live birth requires internal fertilisation

Internal fertilization

In mammals, internal fertilization is done through copulation, which involves the insertion of the penis into the vagina. Some other higher vertebrate animals reproduce internally, but their fertilization is cloacal.The union of spermatozoa of the parent organism. At some point, the growing egg or...

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In a study of fossil remains, comparison of the ontogeny

Ontogeny

Ontogeny is the origin and the development of an organism – for example: from the fertilized egg to mature form. It covers in essence, the study of an organism's lifespan...

 of fourteen dermal plates from Compagopiscis croucheri and the more derived species Incisoscutum ritchiei suggested that lengthwise growth occurs earlier in the ontogeny than growth in width, and that dissociated allometric heterochrony has been an important mechanism in the evolution of the arthrodires, which include placoderms.

These same fossil specimens also show that I. ritchei was a predator, as muscle fibers from the tails of other placoderms have been found in the stomach regions.