Claspers (male sex organs) in living vertebrates (sharks, few fish) are used for internal fertilization and penetrative sex. Most modern fish and amphibians fertilize externally by releasing their
gametes into the water, where fertilization takes place.
M. dicki is the earliest placoderm and vertebrate that shows evidence of internal fertilization. This has been observed in several new specimens that clearly display
sexual dimorphism between males and females in the species. These specimens have been found to either have bone claspers for the males or plates for the females, at the end of their armor plates. Reinterpretation of several more derived placoderms have shown similar structures to the condition in
M. dicki.
Male claspers anatomy The clasper of
M. dicki is a deeply grooved dermal bone that curves laterally at almost a 90 degree angle to the side of the animal. The clasper extends in length laterally from the mid line past the width of the trunk shield. These claspers are fused with the posterior ventrolateral plate and to each other. This would suggest that these claspers were incapable of moving. Additionally' there is a groove located on the ventral surface of the clasper. Current hypotheses suggested that the groove was used for sperm delivery either in the form of a transport channel for the sperm or was used for an encased structure that carried the sperm canal. The male clasper having a width wider than the trunk shield would allow males and females to enter coatis from the side as apposed from other angles. Other placoderms such as ptyctodontid placoderms have similarly shaped claspers. For example, the clasper of
Austroptyctodus gardineri is a dermal bone that curves laterally to the side of the animal and resembles a hook.
Female genital plate anatomy Female specimens of
M. dicki show paired blade-like structures in the same region of the animal that the claspers are in the males. These blades are ornamented with curving ridges and marginal tubercles on their dorsal (internal) surfaces facing the cloacal chamber and are fused to each other and posterior ventral lateral plate as well. The plates are flat and taper to the lateral ends of the transverse ventral ridge inside the lateral lamina of the posterior ventrolateral plate. These blade like structures have been interpreted as female genital plates. These genital plates suggest that they were used as a grip surface for the male clasper to attach to during copulation. Other placoderms such as the antiarch
Pterichthyodes milleri, have identical ornamentation on the dorsal surfaces of the genital plate. Additional, the antiarch
Bothriolepis sp. display semicircular plates located in the same anatomical area suggesting that these specimens are females and structures used for mating.
Differences from other groups Microbrachichiidae is identified as earliest branching placoderm lineage with claspers. Other placoderm groups such as ptyctodonts and arthrodires display evidence of similar innovations. The clasper structure in
Microbrachius is clearly different from the pelvic girdle and pelvic fin. In other more derived antiarchs such as asteolepidoids and bothriolepioids, there is a complete lack of pelvic girdle and fins. In ptyctodont and arthrodire placoderms, the clasper is immediately posterior to the pelvic fin. Arthrodire claspers do not articulate directly with the pelvic girdle or fin. In ptyctodonts, the endoskeleton of the clasper was unossified (unknown affinity to other structures) but away from pelvis. Placoderms differ from chondrichtyans by having their claspers independent from the pelvis and pelvic fin.
Mating behavior One such specimen shows two individuals preserved next to each other in what has been inferred to be side by side mating. As the male and female engaged in coitis, the animals have interlocked claspers to genital plate in a side by side position. Additionally the specimens show the use of interlocked pectoral appendages, possibly to additional help copulation.
Implications New specimens of
M. dicki have started the conversation of which is the primitive state for reproduction in vertebrates, internal or external fertilization? The traditional model is that external fertilization is the primitive state with internal fertilization evolving at different times in different clades. The discovery of mating specimens
M. dicki gives the oldest proof of internal fertilization in jawed vertebrates. This lends credence to the possibility that internal fertilization evolved first and that actinopterygian fish and most lissamphibians (living amphibians) modified or lost these claspers and plates into forms for external fertilization. ==References==