ARTHROPODA

The arthropods are far and away the dominant form of life on earth today, and presumably were so in the past as well. Most lack a mineralized skeleton, which probably accounts for their typically poor fossil record. There are, however, taxa within the Arthropoda with fair to quite good records.

"Arthropoda" means "jointed leg", in reference to the exoskeletons of arthropods with their obvious jointed appendages (both legs and otherwise). In most, the exoskeleton is organic, sometimes phosphatic, but in a few groups the skeleton is calcite. The body is typically segmented into discrete parts, each of which has a specific function. But some primitive arthropods retain a different type of segmentation, in which several segments simply repeat themselves, each performing the same function. This body plan is called "metameric" or "metamerism". Annelid worms, such as earthworms, are metameric, and are thought, therefore, either to be ancestral to arthropods or to share a close common ancestor with them.

Classification of arthropods is much debated. I give one here, but practically any book you examine will have some more or less slight variation on it.

PHYLUM Arthropoda

CLASS Onychophora -- Primitive, caterpillar-like beasts. Known for a long time only from the C Burgess Shale (marine) but fairly recently found alive in Australia (terrestrial).

CLASS Myriapoda -- Centipedes and millipedes. All terrestrial. Very rare as fossils but known from C-Rec.

CLASS Hexapoda (=Insecta) -- Insects. Uncommon as fossils, but known from D-Rec. Terrestrial.

CLASS Crustacea -- Predominantly marine with many body shapes.

•     SUBCLASS Phyllocariida -- rare shrimplike things with organic bivalved shell covering anterior end. C-Rec.
•     SUBCLASS Holocarpida -- Rare shrimpish things, including "mantis shrimp" or "thumb- busters".
•     SUBCLASS Eumalacostraca -- Several orders of rare, varied taxa living in many habitats, but generally with poor fossil          records. The exception is:

                     ORDER Decapoda -- Crabs, shrimp, lobsters, etc. Practically all marine (but, note that crawfish belong here.)

•     SUBCLASS Ostracoda -- tiny shrimp-shaped thingies with a calcite bivalved shell. They are very important because              of their size, which allows them to be recovered from well cuttings. In addition, they live in virtually any aquatic               habitat, and so may be useful where marine organisms are lacking.

CLASS Chelicerata -- Diverse forms as below.

•     SUBCLASS Merostomata -- Marine or freshwater forms of two types:

                     ORDER Xiphosura -- Horseshoe crabs. C-Rec. Marine.

                     ORDER Eurypterida -- Eurypterids. M.O-P. Marine and freshwater.

•     SUBCLASS Arachnida -- Spiders, scorpions, harvestmen, etc. Terrestrial except one Recent group. Rare, but S-Rec.

CLASS Trilobita -- Trilobites. All marine. C-P.

Several distinct Families separated by structure of the pygidium and cephalon (particularly the glabella and eyes), thoracic segmentation, facial suture type, genal and pleural spines, occipital ring, and hypostoma. There are three approaches to lumping them into Orders:

A) The admission of defeat approach, Type I -- "We don't have a clue how to group most of them, so we'll have 2 Orders: one for the agnostids and one for all the others."

B) The admission of defeat approach, Type II -- "Ditto, so we'll have a separate Order for each Family."

C) The "boldly go" approach, with several variations - We think, or we believe, or we have some vague notion that the families relate such that we can create this set of orders ..., followed by a proposed classification.

TERMS TO BE FAMILIAR WITH:

• Axial Furrow -- Groove bounding the axial lobe. (FIG.)
• Axial Lobe -- Central longitudinal raised lobe of a trilobite. (FIG.)
• Cephalon -- Head. (FIG.)
• Doublure -- An under-folding of the dorsal skeleton which edges the ventral surface of a trilobite.
• Facial Suture -- Line along which cephalon splits during molting, allowing trilobite to emerge. (FIG.)
• Fixigena -- Portion of the cephalon medial to sutures, from which librigenae fall free during molting. (FIG.)
• Genal Angle -- Posterior lateral corners of the cephalon.
• Genal Spine -- Distinct spine projecting from the genal angle. (FIG.)
• Glabella -- Raised central part of the cephalon; the anterion end of the axial lobe. (FIG.)
• Glabellar Furrow -- Transverse groove partly or fully across the glabella. (FIG.)
• Gonatoparian -- Condition in which facial suture intersects cephalic edge at the genal angle. (FIG.)
• Hypostoma -- Plate immediately in front of the mouth.
• Librigena -- Lateral portions of the cephalon which break free during molting. (FIG.)
• Marginal -- Condition in which facial suture follows immediately along the edge of the cephalon. (FIG.)
• Occipital Furrow -- Groove on the cephalon immediately behind the glabella which hooks under the first pleura of the thorax, articulating the two parts. (FIG.)
• Opisthoparian -- Condition in which the facial suture intersects the cephalon edge between the midline and the genal angle. (FIG.)
• Pleura -- Part of one of the segments of the thorax lateral to the axial lobe. (FIG.)
• Pleural Lobe -- One of the two lateral longitudinal lobes of a trilobite; flank the axial lobe. (FIG.)
• Pleural Spine -- Spine projecting from the end of a pleura.
• Proparian -- Condition in which facial suture intersects cephalon edge anterior to the genal angle. (FIG.)
• Pygidium -- Posterior segment ("tail") of a trilobite. (FIG.)
• Telson -- Spine on pygidium or the middle of a posterior thoracic segment, pointing directly posterior.
• Thorax -- Middle body segment, between cephalon and pygidium, comprising a number of shorter "thoracic segments". (FIG.)

PALEOECOLOGY -- Arthropods as a whole live virtually anywhere and feed in virtually all possible ways. The ones of most interest to paleontologists (because they are commonly preserved, and not because we don't care about the others) are all aquatic, and mostly marine. We will pay attention to the ecology of two groups -- the ostracodes and the trilobites.

OSTRACODES:

1) Live in virtually any aquatic habitat.

2) Any one species may not be able to tolerate variations in salinity, d.o., pH, or whatever, but some species is probably able to live in water that would kill most organisms.

3) Fossil assemblages characteristic of carbonate tidal flat environments are quite often strongly dominated by ostracodes, because hypersaline water is the rule.

TRILOBITES:

1) All were marine, and it's likely that they all required perfectly normal marine water.

2) They appear strongly controlled by substrate type (grain-size, for example) and so might make excellent environmental indicators. Unfortunately, individual species must be studied individually. There is little general paleoecologic work available for the class as a whole.

3) Agnostids and others which apparently lived in very deep water were eyeless.

BIOSTRATIGRAPHY:

1) Ostracodes as a rule are not terribly good guide fossils. They do get some use for some parts of the geologic column (the Silurian is probably where they're best), and, of course, they may be the only available taxon for correlating deposits of environments with strange (read "nonmarine") water chemistries.

2) Trilobites are exceptionally useful guide fossils in Paleozoic rocks. The initial definition of the Paleozoic was that accumulation of Systems bearing trilobites. It is now clear that part of the C predates the earliest trilobites, but they are still very useful.

3) For parts of the Paleozoic, particularly C-D, good zonations within series or stages can be accomplished with trilobites.

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