Monday, March 12, 2012

Trilobite sexuality

Trilobite with brood pouch

Sex in the Cambrian period?

Were they not all just some organisms multiplying somehow... in a very primitive way?

Sexuality is a concept that involves usually two distinct types of the same organism we usually call in English male and female.

Sexuality suggests pairing of different body parts which is in evolutionary theory a serious complication. Why and how two different creatures must be there in the line of reproduction? How come...

Sexuality suggests much more complex social behaviour than just being there and bringing one offspring after other to this world since it has an evolutionary purpose helping the species to choose attractive partner.

It is quite a world in itself and dominates modern humans.

Well, whatever... The fact is, according to Sam Gon III, trilobites had sex in those Cambrian oceans. The evidence is there in the fossil evidence but apparently quite rare and not so simple to analyse and interpret.

Drawing ©2000 by S. M. Gon after Fortey & Hughes 1998
Trilobites are thought to have reproduced sexually, as do nearly all arthropods today.

Eggs were presumably laid, but fossilized eggs that may be of Cambrian eodiscid trilobites have been documented only once (Zhang, X. & B. Pratt. 1994. Middle Cambrian arthropod embryos with blastomeres. Science 266:637-9)..

It has recently been suggested that some trilobites may have held eggs and/or developing young within the cephalon (as horseshoe crabs do today), and anterior median swellings of the cephalon (of the preglabellar field) in some specimens are interpreted as brood pouches (e.g., Fortey, R.A. & N.C. Hughes. 1998. Brood pouches in trilobites.  Paleontology 72(4):638-49) because they appear only in holaspids (adults) and represent a dimorphism in which the swelling is the only morphological difference.

Ostracods and some other crustaceans show similar brood pouch swellings, although not at the anterior of the body. Because specimens with brood pouches appear only in natant trilobites, it is possible that the eggs or protaspids were released ventrally, anterior of the hypostome.
trilobite info pages

Horse shoe crab 

Horse shoe crabs are sometimes met at the beaches on Earth today.

The animal helps really looks a bit like a trilobite but is not a relative. It helps scientists to understand the features on surviving trilobite fossils like in the case of suggesting that maybe also trilobite young developed within the cephalon.

As for the physical and social details of arthropod sex in general and the love life of horse shoe crabs and trilobites in special... 

Very interesting in many aspects but I skip it for now.

The important point here is the presence of sexuality in these early life forms in Cambrian seas.

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