Introduction

    This week, we begin our examination of fossil arthropods, the jointed-limbed invertebrates that first appear in  Lower Cambrian rocks, and are a dominant group of the famous Chengjiang Biota,  the early Cambrian aged fossil fauna from China, and the Burgess Shale,  the mid-Cambrian aged fossil fauna from Canada. The Arthropod Phylum includes four Subphyla:  the Chelicerates (including arachnids, horseshoe crabs), the Crustaceans (lobsters, crabs, shrimp, barnacles, ostracodes, and others), the Tracheata (the insects, centipedes, and millipedes), and finally, the famous fossil group, the Trilobitomorpha (Trilobites) (Cambrian-Permian).  This week, we will focus on this extinct Subphylum alone, because it is one of the most useful fossil groups to the paleontologist and stratigrapher working in Paleozoic rocks.  The Trilobite group has undergone numerous phylogenetic classification revisions, and at present there are 12 Orders recognized (see your Benton & Harper text here for details on the twelve orders).

Arthropods in General:  Summary Table:
 

Basic Facts to Know about Arthropods:  They are
1.	Eukaryotes
2.	Metazoans with organs, true tissues, nervous, muscular, and reproductive systems
3.	characterized by body segmentation (externally in the exoskeleton, and internally in the nervous, muscular, hemal and excretory systems).
4.	characterized by Bilateral Symmetry, with a distinct anterior (acron) and posterior (telson).  As in annelids, the most recent segment is directly anterior to the telson.
5.	characterized by a rigid Exoskeleton (Cuticle) composed of protein and chitin (a polysaccharide), which must be molted (shed) in order for the animal to increase in size and add additional segments and/or appendages.  The Exoskeleton is strengthened (hardened) through sclerotization (all arthropods) and in addition, mineralization (crustaceans, millipedes, and trilobites).
6.	paired, jointed segmental appendages, typically these appendages are specialized for particular tasks
7.	found in all known oxygenated environments, aquatic and terrestrial, and in some hypoxic (very low oxygen) environments
8.	Excellent index fossils:  widely distributed, well preserved, easily identified, rapidly evolving
9.	found to filter feed, or to be heribivores, predators, scavengers, detritivores, or parasites.
10.	gonochoric (males and females present, with a few exceptions)

Subphylum Trilobitomorpha
Cambrian-Permian

The trilobites are one of the most common benthic marine fossils of the Paleozoic, ranging in size from about 0.5 mm all the way to about 70 cm (the enormous Isotelus trilobites).  The features of the exoskeleton include pronounced segmentation, and a strongly mineralized cuticle, in which the chitin is strengthened by calcite with phosphate.
    Evolutionary patterns among the Orders are very useful in biostratigraphy.  The less specialized families within the Orders Agnostida, Redlichiida, Corynexochida, and Pythcopariida became extinct at the close of the Cambrian, with surviving groups having a variety of anti-predation characteristics, including the ability to burrow, and the presence of spines.  Trilobite diversity dropped again at the close of the Ordovician, with the extinction of the Orders Ptychopariida, Corynexochoida and Agnostida.  Trilobites rebounded slightly in Devonian time, and it is during this time that the Phacopida expanded.  Only two orders survived the Late -Devonian extinction events, the Phacopida and the Proetida.  These two orders continued through late Paleozoic time, although became only a rare component of the marine benthic fauna, and typically associated with reef environments.  The Order Proetida was the last group of trilobites to survive through the Paleozoic, becoming extinct in the Permo-Triassic extinction events.
 
 

Basic Facts to Know about Trilobites:  They are
1.	an extinct Subphylum (Cambrian-Permian) of the Arthropod Phylum (see Arthropod Table above for general features)
2.	marked by a dorsoventrally flattened body with an anterior Cephalon (head), a middle Thorax, and a posterior Pygidium
3.	marked by 2 longitudinal grooves, the axial furrows, divide the body into 3 side-by-side lobes (hence the name Tri-lobe-ite)
4.	classified in part on the basis of Facial sutures on the Cephalon.  Three general groupings:  Proparian, Gonatoparian, and Opisthoparian suture patterns are used.
5.	equipped with Compound eyes having calcite mineral lenses (crystals oriented on the "c" axis), which transmits light to photoreceptors.
6.	known to have eyes that were either Holochroal (many small, hexagonal, tightly packed lenses) or Schizochroal (a few large, spherical, widely spaced lenses).  Holochroal eyes are most common
7.	equiped with Appendages include 1 pair of antennae (usually not preserved) followed by many pairs of segmented appendages down the length of the body, from Cephalon to Pygidium, without regional specialization (except for the antennae and the first pair of appendages after that).  The dorsal coverings (pleurites or pleural spines) over appendages are attached to an axial ring on the axial (central) lobe.
8.	found in marine environments, from deep sea to shallow photic zone,  mostly benthic but some were planktonic.
9.	equipped for filter feeding, scavenging, detritally feeding
10.	equipped with spines, heavy mineralization of chitin, ability to enroll and to dig into the sediment to discourage predation

Part I.  Draw any 4 of the trilobite fossils in the teaching collection.  Label, if visible, the following parts and the Order and Genus, if available, 20 pts.  :
Cephalon:  Label the Cephalon, Eyes, Glabella, Palpebral Lobe, Genal Angle.  Can you tell if this form is Proparian, Gonatoparian, or Opisthoparian?
Thorax:  Label the Thorax, axial ring, pleurites (pleural spines)
Pygidium:  Label the Pygidium, axial ring, pleurites

Agnostid trilobites (Order Agnostida, Cambrian-Ordovician)
Probably planktonic.  Typically have at most 3 thoracic segments.
Image Shown with Permission from the University of California Museum of Paleontology

USEFUL TRILOBITE WEB PAGE LINKS

Click here to see the really excellent Trilobite Web Page by Sam Gon, III, Director of Science, Nature Conservancy, Hawaii

Trying to identify Trilobite moult pieces?  Click here to see the The Cincinnati Trilobite Fragment Identifier, written and produced by Bill Heimbrock.