Trilobites, Paleoecology and Anomalocaris

The “Explosion,” as the Cambrian Diversification is commonly referred, occurred approximately a half-billion years ago and is represented by the rapid appearance of about fifty phyla of animals. Present day scientists through the study of numerous fossils, as well as, genetic research have identified this uprising from the Kingdom Animalia and have come to the astonishing conclusion that the entire event lasted, at the very most, a mere ten million years. Most would agree that this seems an extremely minute quantity of time to permit such an all-encompassing radiation, which is one reason why this event is not only a topic of current research, but also one of heated debate.

A linear representation of animal evolution would certainly have a spike near the geologic time of the Cambrian Explosion. All present day phyla (i.e. body-plans, or animal “designs”) arose during that time (with the exception of Bryozoa) including that of the most abundant animal, and second most abundant organism (following only bacteria) on earth today, the arthropods.

One of the groups to rise from the Cambrian is the Phylum Arthropoda. Today the most common arthropods are the insects, but in pre-history, deep in geologic time, the title of most abundant arthropod may quite possibly have gone to the class Trilobita.

Trilobites are without a doubt one of the most easily recognized fossils in modern times, their abundance and variety have played a key role in paleontology as they act as wonderful index fossils. Variation in trilobites covers a wide range of morphological deviation, but most hold a few key characteristics in common. These common morphological characteristics include the division of the trilobite exoskeleton into three distinct regions, those being the head, (cephalon), the main body (thorax) and the tail (pygidium). These regions, especially the cephalon, have distinct sub-features that aid in the identification of individual trilobites, some of which are diagramed below in Figure 1.

FIGURE 1

Trilobites were structurally similar to many modern day arthropods; they possessed jointed appendages and hard exoskeletons, which fortunately - in conjunction with the process of molting - provided us with numerous high quality fossils today. The trilobites ranged in size from mere millimeters to over two-feet in length. They occupied primarily calm, deep waters were there was an abundance of fine silts which they plowed through with their flattened cephalons in order to search out rich debris to be used as a food source. Some trilobites however were most certainly predators, and many may have occupied other niches as well. They appeared (or they apparently appeared - some evidence suggests that they may have earlier origins) early in the Cambrian, reached their zenith in the late Cambrian and then began diversifying up till the Permian during which time they became extinct. There have been estimates of greater than 20,000 species of these incredibly successful Paleozoic marine arthropods - making classification rather tedious at times with new finds occurring on a regular basis. Luckily, some fossils retain sufficient detail as to render their classification relatively certain; however fossils don’t always readily describe the ways in which organisms interacted within their ecosystems.

This is one reason why Jennifer Dunne, et al, conducted research focused towards delineating the food-webs and niche interactions of species identified from the Chengjiang and Burgess Shales. From the Author’s Summary of Compilation and Network Analyses of Cambrian Food Webs,

“Our analyses show that for most aspects of network structure, the Early Cambrian Chengjiang Shale and Middle Cambrian Burgess Shale food webs are very similar to modern webs. This suggests that there are strong and enduring constraints on the organization of feeding interactions in ecosystems. However, a few differences, particularly in the Chengjiang Shale web, suggest that some aspects of network structure were still in flux during early phases of de novo ecosystem construction.”

In another paleoecology related story, Mariel Schotenfeld from the University of Massachusetts Amherst has challenged the widely held idea that Anomalocaris preyed on trilobites.

Anomalocaris

From upcomming G.S.A. agenda: “The Cambrian animal Anomalocaris is hypothesized to have eaten trilobites and other biomineralized prey. The lack of broken or abraded teeth on the plates comprising examination of the mouth apparatus of Anomalocaris suggests that it may not have had the ability to break the exoskeletons of any hard-shelled animal. SEM – EDS of the mouth apparatus from Burgess Shale specimens, indicate that the 32 plates are composed of organic carbon, suggesting they were originally unmineralized cuticle.

Mechanical properties of these plates were analyzed using CAD modeling and Finite Element Analysis. Poisson's ratio and Young's modulus of potential Anomalocaris plates, as well as density and fracture strength used for the FEA analyses, were estimated using a range of modern-day arthropods. Two end-member values were used both to approximate the range of strengths exhibited by Anomalocaris' cuticle, and also to encompass the range of exoskeleton strength likely exhibited by trilobites. The hardest skeletal values are from wet lobster (Homarus americanus) crusher claw cuticle; these are most likely to deform in a brittle manner. The softest are from adult dung beetle (Copris ochus) cuticles. In order to bite and successfully break the calcified cuticle of a trilobite, Anomalocaris' mouth plates would have needed to withstand forces that are greater than those required to fracture a trilobite exoskeleton. Results demonstrate that the teeth-like structures of the mouth plates should have deformed or broken when less than 90 N of force was applied perpendicular to the plates.

Additionally, documented trilobite malformations were compared to modern and extinct arthropod malformations. Abnormal trilobites previously attributed to predation of Anomalocaris might also be interpreted as molting failures or genetic mutations; such malformations occur with similar frequency in modern marine clawed lobsters, brachyuran decapods, and limulids. Furthermore, there is no direct evidence for Anomalocaris' feeding habits such as gut contents.”



Dunne, J., Williams, R., Martinez, N., Wood, R., & Erwin, D. (2008). Compilation and Network Analyses of Cambrian Food Webs PLoS Biology, 6 (4) DOI: 10.1371/journal.pbio.0060102