Chondrichthyan fish have cartilaginous skeletons - in contrast to osteichthyan (bony) fish - and comprise the Elasmobranchii (sharks, skates and rays) and the Holocephali (chimaeras). Whilst vivparity (giving birth to live young) dominates as the reproductive strategy in modern day Elasmobranchs, the Holocephali are exclusively oviparous (egg layers). The authors suggest that oviparity is the likely ancestral state for cartilaginous fish, whilst viviparity is assumed to have evolved several times in the Chondrichthyan lineage.
Chondrichthyan egg capsules are currently categorised into 10 morphotypes, with 8 of these types known from the fossil record. Using cladistical analysis of morphological traits of the egg capsules, the researchers attempted to see if the relationship would yield a phylogenetic signal.
They found that a very strong Elasmobranch group was generated, with very clear morphological similarities in the egg cases of Heterodontus, Orectolobus, Scyliorhinid and Rajoidae. The Holocephalan egg cases showed a greater similarity to the egg cases of extinct species than modern day Elasmobranchs.
The results suggest that the Elasmobranch egg capsule is the derived state and the ancestral reproductive state in both live bearing and egg laying jawed vertebrates is internal fertilization paired with clasper organs.
Jan Fischer, Martin Licht, Jürgen Kriwet, Jörg W. Schneider, Michael Buchwitz & Peter Bartsch (2014) Egg capsule morphology provides new information about the interrelationships of chondrichthyan fishes, Journal of Systematic Palaeontology, 12:3, 389-399,
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