REPRODUCTIVE STRATEGY OF METRIORHYNCHID CROCODYLIFORMS: A COUNTER-INDUCTIVIST ARGUMENT

FERNÁNDEZ M.S.; LAMAS, S.G.; HERRERA Y.

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

A major concern when testing scientific hypotheses in general and palaeobiological hypotheses in particular, should be whether it is more accurate to find cases that confirm them or cases that disconfirm them. The former is basically an inductivist approach. The latter is counter-inductivist, and is the approach that which will be used herein. To this end, a case study on the reproductive mode of Jurassic metriorhynchids (Crocodylomorpha, Archosauria) has been selected in order to evaluate available arguments for testing two alternative hypotheses: egg laying (oviparity) and the bearing of live young (including egg retention and viviparity). It must be noted that no pregnant metriorhynchid female or egg containing an embryo have been found. Traditional and recent reviews of the reproductive strategies of reptiles (including Mesozoic marine forms) avoid dealing with the second hypothesis and assume oviparity to be the only reproductive strategy for archosaurs. The oviparity hypothesis for metriorhynchids is principally based on their phylogenetic position within crocodylomorph archosaurs, and on comparison with living forms. As all living archosaurs (birds and crocodiles) are oviparous and some extinct archosaurs (i.e. dinosaurs and pterosaurs) were oviparous, it is generalized that all archosaurs (extinct and extant) were oviparous. However, certain aspects of these peculiar crocodyliforms have been overlooked and, according to a counter-inductivist approach, should be analyzed, as they contradict the oviparity hypothesis. These aspects are related to the peculiar body plan of metriorhynchids, including forelimbs that are transformed into paddles, femur and zeugopodial hindlimb bones that are relatively flattened with rounded articular surfaces, the osteoporotic-like bone tissue of the femur, and a hypocercal tail. All this evidence seems to contradict the possibility of the body being supported outside the aquatic medium. On this basis, consideration should be given to other possibility, that is, that metriorhynchids were not only marine nektonic but also obligatory aquatic forms. In this case, oviparity seems unlikely. Gas diffusion is considerably slower in water. If eggs were laid underwater, oxygen deprivation (hypoxia) would have seriously affected embryonic development. Experimental studies on aquatic reptiles that lay their eggs in seasonal ponds have demonstrated that the embryos remain in developmental arrest as long as they are immersed in water, and resume development only after the soil dries and oxygen tension rises. Then, if it is accepted that metriorhynchid crocodylomorphs were not only marine pelagic but also obligatorily aquatic, the live-bearing hypothesis could be accepted.