The question of feeding habits and food-gaining strategies in trilobites can only be addressed by using an indirect approach, as we are dealing with an extinct life form without any direct descendants, known only from fossilized remains (see: Reproduction). There are but a few indications, the analysis of which allowing for some more or less obvious conclusions. In general, we have to base our investigations on the following circumstances:
• Trace fossils, e.g. Rusophycus and Cruziana, which do not represent the actual remains of trilobites but the tracks they left in originally soft sediment which lay undisturbed until it was covered by other layers of mud and thereby preserved as a trace fossil. Such tracks of movement can be used to construe behavioural patterns.
• The morphological aspects of the trilobite head shield, in particular the shape and size of the glabella, which seems to have contained a stomach, and the shape of the cephalic border.
• The presence of strongly articulated so-called gnathobases, a particular pair of legs situated close to the point where the trilobite’s mouth apparatus should be expected, beset with rows of “spines” or “teeth”.
• The orientation of the hypostome, its shape, size and type (conterminant, natant, impendent). The hypostome seems to have played a significant role in the intake of food, with the opening of the mouth sitting right above its posterior border. It may have served as a kind of “ramp” to feed in food particles.
• The size of the holaspid or adult trilobite itself, as it seems more likely for larger taxa to have played a predatory role in local ecologies, taking into account the overall fauna as we know it from the fossil record.
Trilobites displayed an extraordinary diversity in reference to their outer shape. Morphological complexity continued to increase throughout the Cambrian and Ordovician and was still notable up to the Carboniferous. It is not unusual to encounter up to twenty different trilobite species within the fossil record of a single biofacies, differing considerably in their morphological aspects. This seems to support the thesis that there must have been a wide range of potential food sources that allowed all these individual taxa to live alongside each other without the need of competing directly for nutrition.
FORTEY & OWENS, in their 1999 paper (Feeding habits in trilobites, Palaeontology, Vol. 42, Part 3, pp. 429-465) concluded that it was possible to extract important information from the combination of different characteristics and proposed to classify trilobites into several groups of eaters:.
• Predators and scavengers
• Particle feeders (ingesting free or rock-bound organic remains of animal or herbal origin)
• Suspension feeders (ingesting organic material extracted from the sediment by rummaging through it and sorting out edibles)
• Filter feeders (filtration of edible parts directly from the water column or intentionally disturbed sediment by engaging the cephalon and extremities in the process
We cannot and do not want to discuss the entity of conclusions as brought up by Fortey and Owens, in particular the parasite option which seems to be a very special one, but would like to concentrate on some basic patterns easily comprehensible.
The hypostome in particular receives increased significance in the process of nutrition. Fortey and Owens emphasize that it has to be regarded as being part of the mouth apparatus, assisting the animal in feeding in edibles. Both recent and ancient predators and scavengers, the latter being known from fossilized remains only, usually display a mouth apparatus designed to both grab as well as hold prey, break and tear it up until it matches the capacity of its ingestive system. This asks for a certain stability and construction. As already mentioned, there are currently three different types of hypostomes that can be found in trilobites. Conterminant and impendent hypostomes are rigidly fused with the rostral plate (even if separated by a suture) or, in rare cases, directly attached to the anterior part of the cephalic doublure which is regularly a very strong construction and resilient basis (e.g. Asaphus). Such hypostomes frequently display additional peculiarities in shape resembling forks, teeth, in some cases rasp-like protrusions and the like. And it is interesting indeed that most species showing these characteristic hypostomes belong to the larger taxa within the trilobite class, usually with very convex glabellas. This and other observations lead Fortey and Owens to believe that trilobites with conterminant or impendent hypostomes have to be regarded as predators or scavengers which were able to hold and feed in food by aid of their gnathobases and hypostomes.
Fortey and Owens regarded predators and scavengers as following a more primitive, primordial feeding habit, although carnivorous feeding seems to have continued way beyond the Ordovician and can be found in many prominent taxa like the Paradoxidoidea, Corynexochida, some Asaphida, Lichida, Odontopleuridae, Calymenoidea, Phacopina and secondary conterminant Proetida. Trace fossils, in particular Rusophycus which is generally attributed to larger Asaphids, show distinct impressions of fork-shaped hypostomes and gnathobases. Such tracks often converge on others originating from what appears to have been worm-like soft-bodied creatures. This seems to add to the conclusion that these trilobite taxa preyed on other animals. However, this interpretation has been doubted, and it remains to be seen whether it can be supported by additional evidence in the future (RYDELL et al., 2001: Trace fossil associations in the Swedish Mickwitzia sandstone: Did trilobites really hunt for worms? GFF, Vol. 123 pp. 247-250)
Unlike conterminant and impendent trilobites, natant taxa, that is trilobites in which the hypostome is neither attached to the rostral plate nor fused with the cephalic doublure, show an extraordinary continuity in hypostome shape. Throughout the 300 million years of trilobite reign, changes were but marginal. In natant taxa, the hypostome is sort of free-floating and only surrounded by more or less soft tissue, unsuitable to be used to grasp struggling prey or process solid food in the way predatory trilobites could. On the other hand, this flexibility might have served the animal as a steered sort of “flap” that could be lowered and retracted as necessary in order to function as a ramp to feed in edible particles from the surrounding sediment. Trilobites following this feeding habit usually belong to small-sized taxa and are often found in large clusters. A good example is the well-known Elrathia kingii, specimens of which can be found in almost every trilobite collection. On the one hand, these small animals are placed at the lower end of the food chain (the famous nutrition pyramid) and have to even the odds of survival by producing much more offspring than their predatory cousins, on the other hand they had access to large resources of food as the sediment must have been abundant with organic remains of one form or another. Trace fossils like Cruziana are generally attributed to this type of trilobites, the tracks of which showing distinct bilateral drag marks from its genal spines impressed deeply into the sediment, indicating a semi-benthic harvesting.
According to Fortey and Owens, trilobites displaying natant hypostomes therefore represent particle or suspension feeders, harvesting edible organic remains by either grazing them directly from the sediment’s surface or filtering, using their “hypostome elevator” as a tool to feed in. Their frequent appearance as mass assemblages in the fossil record in combination with their overall small size seem to support the idea of a non-predatory life mode in this particular group.
Apart from predators, scavengers, particle and suspension feeders ist is the filter feeders which form a specialized group within the class trilobita. These trilobites regularly display a very large fringe, in most cases closely pitted. Examples can be found within the genera Harpes , Onnia and others. It is supposed that their mouth apparatus were situated way above where other trilobites used to have it and that they featured a sort of filtering chamber within the cephalic shield in which edible food particles could be sorted out. According to Fortey and Owens, the typical filter feeder morphology is one in which the convexity of the cephalon considerably exceeds that of the thorax and tail shield. They also see support of their theory in the fact that in some suspicious taxa the hypostome shows a very steep orientation (Illaenus, Symphysurus, Ampyx ) with the posterior part at far distance from the sediment. Assuming that the mouth apparatus in these taxa were placed like in all others trilobites at the posterior end of the hypostome, it is difficult to imagine how these animals were able to gain their food other than by filtration. Not only is the mouth apparatus too far from the substrate to directly take in food particles but the steep orientation of the hypostome reders it useless in grasping and holding prey or processing other animal remains.
It has to be said that there are a great many examples in which this combination is not evident, the animal, however, still being regarded as a filter feeder. We won’t go into discussing this matter in detail but would like to summarize the findings of Fortey & Owens as follows: trilobites with conspicuously broad cephalic fringes, pitted or not, in combination with a very convex glabellar morphology may be regarded as filter feeders, in which a “nutrient solution“ was processed inside a filtering chamber. The nutrient solution being acquired directly from the water column or the substrate, the latter being intentionally disturbed to release edible particles contained therein. Pitted fringes may have allowed water to flow into and out of the cavern built by the cephalic shield and release the indigestive remains of the substrate back to the environment.
As with many other aspects the Agnostida seem to be problematic insofar as they seem to have lead a pelagic life within the water column rather than on the sea floor. There is an ongoing discussion as to how these trilobites acquired their food and it is way beyond the intention of this website to consider all the pros and cons of the different theories dealing with this subject. The same applies to other trilobites with a similar mode of life as unfortunately we know nothing of the extremities of non-Agnostid pelagic taxa. In all likelihood they fed on plankton (tiny free-floating organisms) or small plankton-eating animals. However, such considerations remain highly speculative and we restrain ourselves from drawing further conclusions.
Another problem can be found in the Cheiruroidea. Trilobites within this taxon feature a conterminant hypostome, but it is distinctly inclined and usually shows a notable granulation. It does dot display any fork-like protrusion as should be expected in a predator and its evolutionary development can only be described as “conservative”. No recognizable change in its morphological layout has been observed in the fossil record covering millions of years. These trilobites may have had a very specialized food-gaining strategy – as of now we don’t know which.
In any case trilobites must have occupied every available ecological niche within the habitat and the question of their feeding habits is but another riddle – the solution to which certainly being one more reason for their outstanding evolutionary success.
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