Environment
Trilobites had an advantage over adapting to new environmental and predatory changes – their hard exoskeleton allowed them to enroll themselves for protection against predators and sudden unfavourable environmental conditions (Lieberman and Karim, 2010). These arthropods occupied a variety of different ocean environments such as shallow flats, deep ocean bottoms, and water columns. Their occupancy in the range of environments is associated with their body characteristics, where trilobites with sight and narrows bodies are adapted to swimming in the open ocean, while eyeless trilobites with wide bodies are adapted to scavenge the sea floor (Lieberman and Karim, 2010). In addition, trilobites were capable of exhibiting a wide range of behaviours; they were swimmers, burrowers, particle feeders, filter feeders, and even predators (Lieberman and Karim, 2010). Despite this, a majority of trilobites were benthic organisms; they crawled on the seafloor, or within complex reefs, and dug into the sediments to search for food, or to conceal themselves from predators (Lieberman and Karim, 2010).
Graphical recreation of the Cambrian sea (A Collection of Cambrian Fossils, 2016).
Interspecies Relationships
Typical predators of trilobites were larger in size such as the nautiloids, predatory worms, sea scorpions and crustaceans (Gon, 2013). Two special cases of relationships are the predator-prey relationship between the Anomalocaris and the symbiotic relationship with Olenimorphs.
Anomalocaris sp. is described as the “trilobite hunter” or “bane of the trilobites” as it was one of the largest predators of trilobites (Gon, 2013). This creature is identified by its large size and anterior raptorial appendages (Nedin, 1999). This creature was able to swallow small trilobites whole and if not, just take large bites out of prey with its ring-like mouth structure composed of sharp-like projections (Gon, 2013). This is evident in many trilobite fossils discovered as it had bite marks or parts missing (Nedin, 1999).
Picture of Anomalocaris sp. a common predator of trilobites
Evidence has also been found on the symbiotic relationship between Olenimorphs and trilobites. Olenimorphs are sulphur-eating bacteria that live in the gill structures of some deeply benthic trilobites (Gon, 2013). The Olenimorphs eat the sulphur-rich environment and produce nutrients for the trilobites to absorb whilst the trilobites provide a home in their gill structures for these Olenimorphs to live (Gon, 2013).
Wujiajiania sutherlandi lived in the dark, anerobic benthos and may have derived much of its nutrients from symbiotic sulphur-eating bacteria that were housed in thoracic gill filaments (Gon, 2013)
Hunting and Feeding Behaviours
Trilobites were the most dominant group of arthropods during the Cambrian and Ordovician time. Their range in modes of life and habitats all aid in their respective trophic groups. The trilobites could be split up into two main groups when describing their feeding habits and these are pelagic and benthic trilobites. Another way of distinguishing trilobites based on its feeding habits is on the type of hypostome it has. Hypostomes are hard mouthparts on the ventral side of the head that can be classified based on its attachment to the rostrum and alignment with the glabella (Fortey, 1990 and Gon, 2013). The two main classifications are natant and conterminant hypostomes. Natant hypostomes are not attached to the rostrum and aligned with the glabella whilst the conterminant hypostomes are attached to the rostrum and also aligned with the glabella (Fortey, 1990). A third class is the impendent hypostomes, which like the conterminant are also attached to the rostrum but not aligned with the glabella (Fortey, 1990).
Figure showing the different types of hypostome attachments. these types of attachment help classify trilobite species (Gon, 2013).
There are two feeding habits that typical pelagic trilobites would be associated with: predation/scavenging and particle feeding. Predatory trilobites were often linked with scavengers as when given the right opportunity, these hunters will scavenge. Typically larger trilobites were associated with being predators. Common physical characteristics of predatory trilobites include a conterminant hypostome, and larger spiny gnathobases which were used to aid in the exterior processing of prey (Gon, 2013). They would also have long legs that would help in moving the processed prey forward anteriorly towards the mouth (Gon, 2013). Common prey would be small invertebrates and mollusks such as worms and clams. A specific example would be the Limulus sp. which was capable of breaking small clams with its gnathobasic jaws (Fortey and Owens, 1999). Another example would be cases in Carolinites sp., Telephina sp. and Opipeuterella sp. in which these predatory trilobites also had larger eyes and a streamlined body to aid in hunting swimming zooplankton (Fortey, 1990). These hunting habits are also common in crustaceans and dominant in Arachnida (Fortey and Owens, 1999). The second feeding habit that pelagic trilobites would have is particle feeding. These trilobites tended to have natant hypostomes which helps it to scavenge for benthic detritus as well as grazing on beds of algae (Fortey and Owens, 1999). These trilobites were often characterized as sluggish and had reduced sizings in many parts as it adapted to its lifestyle and habits (Fortey and Owens, 1999). An example of a particle feeder would be Hutchinsoniella sp. which feeds using its ventral median line as a food groove (Fortey and Owens, 1990).
The rest of the feeding habits are all exemplified through the benthic trilobites. Suspension/filter feeding was a habit in which many trilobites extracted edible food particles from sediment or water. Typically these trilobites were characterized as having elevated hypostomes, which created a space for a filter chamber as well as flatter appendages that helped push sediment or water into its filter chamber to extract the food particles (Gon, 2013). An example of a filter feeding trilobite is the Triops sp. that stirred up clouds of sediment under its dorsal carapace and passed edible particles sorted from suspension into its mouth (Fortey and Owens, 1999). Some predatory and particle feeding trilobites can also be benthic as well.
Figure of a line drawing of a filter feeding trilobite, burrowing into sediment and showing the circulation pattern entering over the gills and exiting through small holes called pits (Fortey and Owens, 1999)
A special type of feeding habit that was considered uncommon was being parasitic. Some small trilobites would adopt homes on fish and larger arthropods and exhibit parasitic habits (Fortey and Owens, 1999). This is mostly seen in the Agnostoids sp.