It is not easy to answer the question of how trilobites reproduced, as we know next to nothing about it. The trilobite class went extinct without any direct descendants from which we could draw conclusions. Our entire knowledge is based on fossilized remains, and usually it is only the hard carapace which is preserved. The outer shell can tell us little about the soft tissues it once protected. As humans, we naturally tend to automatically compare extinct arthropods to recent taxa and the results merely add to the whole range of potential answers, none of which can be checked as “verifiable”.
We can assume that trilobites, being marine arthropods, were oviparous animals. The main question here is egg fertilization. What was the process and did trilobites undertake brood care? There is some variation, for example, in the reproduction of recent crustaceans and how they care for their offspring. Although a few are hermaphroditic, most crustaceans have separate sexes. In some cases the female simply releases its eggs into the open water and the male follows suit by releasing its sperm. Most of the time, though, the male uses a specialized pair of appendages on its abdomen, the so-called "pleopods", to transfer its sperm to the female. In some parasitic taxa, the female actually hosts a very small male (dwarf male) right in its enlarged orifice. The last option surley is the most unlikely as far as trilobites are concerned but the other ones are worthwile to be thoroughly considered.
Up to this time we have never observed any indications in the fossil record that trilobites might have had specialized appendages like the ones described in modern crustaceans. If they actually existed, they must have been of the same condition as the rest of the trilobite extremities. Not enclosed in any mineralized shell, they hardly stood a chance of preservation and there are only a few places in the world where the soft tissue of trilobites left traces in the rock. I am not aware of any trilobite worker ever coming up with something that could reasonably be looked at as being the remains of a reproductive organ.
It would appear that the most likely option is one in which the trilobite simply released its eggs into the environment, either by placing them in burrows within the sediment or attaching them to coral, sponges or marine flora with the male simultaneously releasing its sperm. Or both sexes simply released their parts at the same time into the open water, coordinated by some yet unknown “switch” which could be of a chemical or otherwise recognizable nature (water temperature, intensity of light, moon phases, etc.) I sometimes wonder whether the impressive clusters of fossilized remains that can be found are actually what we usually regard them to be. Are they really death assemblages or are they perhaps mating assemblages?
FORTEY & HUGHES in 1998 suggested that trilobites may have undertaken an active brood care by retaining fertilized eggs in what they believed to be some sort of “brood pouch” until they hatched into larvae. (FORTEY, R.A. & N.C. HUGHES, 1998. Brood pouches in trilobites. Journal of Paleontology 72 (4) : 638-49). This assumption was based on some very rare observations of a massive bubble in the preglabellar area in some Cambrian and Ordovician trilobites. This bubble seems to occur only in adult specimens and was noted as being the only morphological peculiarity within a fossilized group of several individuals of the very same species.
Similar inflations can be found in Ostracods (Ostracods are small crustaceans, typically around 1 millimetre (0.04 in) in size. Their bodies are flattened from side to side and protected by a bivalve-like, chitinous or calcareous valve or "shell") and some other arthropods. The question is: if those bubbles are actually brood pouches, should we not have found many more of these bubble-headed trilobites? Why did we find them in trilobites with a natant hypostome only? What about the conterminant and impendent taxa? Did they conduct a different brood care, if at all? Questions after questions!
One thing that kept on bugging me personally about the brood pouch theory is this: If this bubble is actually what Fortey & Hughes think it is, it should be assumed that it is but a temporary morphology which should recede once the eggs are hatched. If not, our collections should actually swarm with such specimens.
As a general rule, brood-caring arthropods do everything to shield their offspring from any possible danger in order to guarantee propagation of the species. It would therefore be reasonable to assume that a brood pouch in which the animal retained its fertilized eggs may have been just as well protected against environmental hazards or predators as the rest of the trilobite’s dorsal shield – that is to say it needed to be mineralized. And it was, as otherwise we would not have been able to find such specimens in the fossil record in the first place. So far, so good.
At first glance, „temporary“ and „mineralized“ seem to be mutually exclusive. We usually think of morphological changes in the trilobite carapace in terms of regular moulting, a relatively short period in which the trilobite sheds its old shell which has become too narrow for the growing animal and builds a new and larger one. Arthropods superficially “grow” by moulting and therefore in more or less regular intervals rather than continously as vertebrates do. It is hard to imagine how a mineralized brood pouch can develop or recede within this short period of time. Hard to imagine, nevertheless possible and in a short moment I will let you know why.
If we conceded to the theory as true that trilobites could only grow during moulting we would also have to assume that the trilobite’s “reproduction program” was already running when it commenced a regular moulting process. For surely the animal needed to start developing its brood pouch right away once the old carapace was shed in order to achieve the necessary size before mineralization began to harden the bubble.
A potential solution to this riddle – one that appears to be supportive of the brood pouch idea of Fortey & Hughes – may be found in reviewing the moulting processes in some recent arthropod taxa. In some species a mating-coupled moulting in the female is jump-started by the male transferring its sperm to the female. This moulting occurs independent of the regular moulting interval. In the course of this process, it appears possible that the animal was able to inflate a brood pouch by means of lymphatic pressure rather quickly. And from there it is just one small step to imagining the animal to commence another irregular moulting to rid itself from the bubble, for example after it finally released its hatched larvae into the open water. Just a theory but one worth considering.
In their 1998 work, Fortey & Hughes clearly stated that their theory was built on grounds of a limited number of species only and listed 70 candidate taxa from the Cambrian and Ordovician in which brood pouches may be observed. They also concluded that even if their theory be true, brood pouches do not appear to have been a class-wide character.
On the other hand, they do not fail to make reference to the recent horseshoe crabs (Limulidae), in particular to Tachypleus rotundicaudus, an animal receiving attention by Fortey’s description of its culinary aspects in Trilobite! – Eyewitness to Evolution. These animals retain their large, yolky eggs inside a cavern within their cephalic shield (the so-called “Prosoma”) without developing any sort of visible bubble. If we read between the lines, the authors seem to suggest that other trilobites may have carried their eggs in a similar way and that the development of a visible brood pouch must not be regarded as a precondition for the trilobite to undertake brood care likewise.
While working on this web site I took the liberty of reviewing some of the problems I had with the brood pouch idea within the framework of the Yahoo! Trilobites2 Group . And I am grateful that Richard Fortey deemed it worthwile to give an explanatory comment:
"As one of the perpetrators of the brood pouch idea, I ought to chip in at this point to say that no, of course most trilobites didn't have them. The most striking example was described in detail by myself and Bob Owens in Palaeontology 40:p. 451 in an olenid. I think it likely that this curious anterior structure was most often developed in trilobites that lived in oxygen poor environments. I think they may have been associated with larger, 'yolky' eggs (rather as in limuloids), where the trilobites might have needed to give their babies a helping hand during the earliest phases of growth. I also brought up the idea that some of these partiuclar trilobites may have 'farmed' colourless sulfur bacteria in this peculiar habitat, to which not other trilobites were well adapted. We don't know that much about the growth of the structure, although we do know that in the olenids at least pouches were not present in small individuals, so presumably appeared when they were ready to breed. It would be good to know whether they appeared at a particular moult."
In view of the rare finds of trilobites showing the preglabellar bubble the brood pouch idea as brought up by Fortey and Hughes surely cannot be regarded as beyond any reasonable doubt, nor was it the intention of both authors to claim that their theory was more than a theory and that brood care in trilobites was a verifiable fact. I simply believe that in view of the innumerable number of trilobites populating the sea floors of the world’s ancient oceans throughout the Palaeozoic there should be a lot more of fossils unearthed showing this morphological peculiarity, even if it was restricted to a certain amount of species. It should also be said that Fortey & Hughes considered the probability that trilobites may have kept their brood pouches throughout several succeeding moulting cycles. If that be true, every such moulting process would have left another excuviae with a bubble which makes the lack of adequate amounts of fossilized remains appear even more dubious.
We just don’t know enough! To the best of my knowledge there was only one case in the whole history of trilobite research in which trilobite eggs (at least they were regarded as such) have ever been identified in the fossil record – unfortunately within the already problematic Agnostida of the Cambrian (ZHANG & PRATT, 1994). And we know next to nothing about embryonic development either. Having said that, the approach by Fortey & Hughes remain to be a very important contribution and their conclusions need to be addressed in all sincerity.
The question of reproduction in trilobites remains to be treacherous ground and trilobite workers must not be blamed for inconsistencies and discrepancies in explaining this process. They are kept in restraint by the burden of several hundred millions of years between “them and us”. We can only hope to find additional evidence to either verify or refute our ideas and beliefs. Until then we can only speculate on grounds of what we can extract from the fossil record, whether we like it or not.
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