Trilobite Exoskeleton

chitinTrilobite Exoskeleton Origins

One of life's most important evolutionary adaptations was the emergence of the external skeleton, or exeskeleton, presumably in the earliest part of the Cambrian known as the Cambrian Explosion. It is not a coincidence that the early Cambrian marks the beginning of the "macroscopic fossil record". That trilobites first appear in the fossil record already highly diverse and spread geographically is precisely due to its readily preserved, hard external skeleton. Most trilobite fossils are, in fact, the preserved exeskeletons. Only under rare circumstances have the soft body parts of trilobites (and other arthropods) been preserved as fossils. The Precambrian fossil record is extremely poor, and many putative precambrian fossil equivocal at least partly because that lacked hard parts.

There are different hypotheses on the origins of the arthropod exoskeleton in the Precambrian. There is concensus that Arthropoda is monophyletic, and that the Onychophorans (velvet worms) and Arthropoda are sister groups descended from annelids (phylum Annelida), and a member of Superphylum Ecdysozoa, a clade (Aguinaldo, 1997) with a common ancestor comprising animals with a cuticle that periodically molted as the animal grows. Interestingly, the Onychophorans, besides leg having similar structure, locomotion, and embryology similar to myriapods (Anderson, 1973), have exoskeletons. Ribosomal RNA analysis of velvet worms and other arthropods also suggests close affinity between them (Ballard et al. 1992). The velvet worm exoskeleton is a one mm thick chitin and protein layer resembling that of some arthropods. Without a doubt, arthropod phylogeny is problematic, but that the trilobite exoskeleton has origins in Phylum Onychophora is plausible conjecture.

Trilobite Exoskeleton Prosopon

Prosopon are features on the trilobite exoskeletons that are usually smaller and include nodes, perforations, pitting, and pustules, and generally exclude larger features such as pleural spines. In general, the purposes of prosopon as evolutionary adaptations features remain equivocal. They have some use as diagnostic features in identification and classification, but caution must be used as proposon attributes are in a broad sense shared across all of Trilobita. The ubiquitous nature of the prosopon suggest they arose independently and multiple times in different orders exposed to the same environments and selective pressures, an example of parallel evolution.

TuberclesArctinurus Lichid Trilobite Exoskeleton is covered with tubercles

Lochovella deckeri with granulose glabellaTubercles are found in many trilobite families and species, and are particularly common and pronounced in orders Lichida (see Artinurus boltoni) and Phacopida. Tubercles are also prevalent among Proetids (see Proetus tuberculatus morocensis , a trilobite actually named the tubercules on its cephalon. Why tubercles evolved is uncertain, but scientists have conjectured they may have provided a measure of camouflage, especially when combined with color patterning. Similarly, tubercles may have made it more difficult for tentacled predators such as nautiloids to get a firm grasp on them. Tubercles vary in morphology small dome-like nodes to short spikes. Tubercles, especially smaller ones, are often called granules, and terminology such tuberculose or granulous exoskeleton will be seen.


Examples of Trilobite Exoskeletons Features

Color pattern preservation Putative camouflage coloration  Extensive tubercles covering exoskeleton  Trilobite named for cephalon tubercles
Greenops barberi Phacopid Trilobite
Bellacartwrightia whiteleyi 
Arctinurus boltoni
Tuberculose Proetid Trilobite
Suborder Phacopina
Family Acastidae Hamilton Group, New York
Suborder Phacopina
Family Acastidae
Hamburg Group, New York
Order Lichida
Family Lichidae
Rochester Formation, New York
Order Proetida
Family Proetidae
Foum Ziguid, Morocco 
Granulous glabella with many small tubercles
Granulous glabella with many small tubercles
Suborder Phacopina
Family Phacopidae
Haragan Formation, Oklahoma