Introduction

     The Arthropod Phylum includes four Subphyla:  the Chelicerates (including arachnids, eurypterids, and 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 continue our exploration of the Arthropods with an examination of the Crustaceans and Chelicerates, and a cursory look at the Tracheata.  Basic shared characteristics of arthropods are listed in Table 1 below.
 

Table 1:  Shared characteristics of Arthropods

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)

The Crustaceans appear in the early Cambrian fossil record in the fossils of the Chengjiang Biota and in the mid Cambrian Burgess Shale fauna.  Of the many Classes of Crustaceans, the most well known groups in the fossil record include the Class Malacostraca (crabs, shrimps, lobsters, krill, pill bugs, amphipods) and the Class Maxillopoda (ostracodes, copepods, barnacles, plus others).  A feature of the Crustaceans is the adaptation of body segmentation and appendage specialization to every conceivable aquatic environment (besides inhabiting all nonmarine aquatic environments, crustaceans hold the record for living in the highest water temperatures known for metazoans, and also inhabit deep aquifers and the cold abyssal depths of the ocean). As with all arthropods, crustaceans must molt their shells (a process known as as ecdysis) in order to grow.  The molted pieces are often fossilized and occur as common fossils in the marine sediment record and in the records from lacustrine basins.   Body segmentation varies within the Crustacean group.  In many cases, you should be able to identify the head, thorax and abdomen of the organism, as in the lobsters and shrimp, whereas in other groups you will notice reduction in body segmentation, as in the case of the Brachyura (true crabs), where the abdomen is reduced to just a few segments visible on the underside of the carapace.  Because soft parts are not commonly fossilized, the shells are typically the only record we have of crustacean populations.  This is particularly the case with barnacles and ostracodes (both of class Maxillopoda).
 

The Chelicerates appear in Ordovician time (although marine arachnid fossils may have been identified in Cambrian rocks).  The chelicerate body is divided into two sections, the prosoma (cephalothorax) fusing the head and thorax, and an opisthosoma (abdomen).  There are pairs of appendages on all six segments of the prosoma, with the first pair of appendages modified with chelicerae (tiny claws) which are the mouthparts.  The second pair of appendages are the pedipalps, which may be modified to serve as mouthparts or for grasping prey, and then the remaining four pairs are walking legs (hence the 8 legged spiders).

Part I:  Draw 4 specimens of crustaceans or chelicerates in the teaching collection.  Is it a crustacean?  If so, label the Class, and the Subclass or Order if available, and the time range.  Label the head, thorax and abdomen if preserved.  Label the appendages (walking, grasping?) In the cases of barnacles and ostracodes, draw and label the shells.  Which way is top (dorsal) which way is bottom (ventral?)   20 pts.
   If you choose a chelicerate, label the prosoma and opisthosoma, and the Class, the Subclass or Order if available, and the time range.

Images of Arthropods

Class Malacostraca, Order Decapoda, Devonian - Recent

Stenopus hispidus - banded cleaner shrimp.  Image ID: reef0285, The Coral Kingdom Collection, NOAA
Location: University of Hawaii Institute of Marine Biology
Credit: Photo Collection of Dr. James P. McVey, NOAA Sea Grant Program

Class Maxillopoda, Subclass Cirripedia (barnacles) Ordovician-Recent

image shown with permission from the University of California Museum of Paleontology

Class Maxillopoda, Subclass Ostracoda (Ordovician - Recent)

    The ostracodes are microcrustaceans that produce a hinged calcite shell, often ornamented, in which they can completely close themselves up.  They live in every aquatic environment, from marine to freshwater to groundwater to wetlands, with the exception of very acidic water.  They are slightly larger than the forams (typically 500-700 microns), and can also be used as both paleoecological and geochemical data in paleoenvironmental and paleoclimatic reconstructions.  The shell is commonly preserved, although often disarticulated into two valves.  When examining these bean shaped valves, look for the adductor muscle scars.  Other scars are sometimes visible, including the dorsal muscle scars.  Ostracodes are sensitive to temperature and salinity in the deep ocean, and are used to map the changing position of water masses through time.  In fresh water environments, ostracodes are sensitive to water composition as well as temperature.  Their rapid speciation, wide distribution, and commonly preserved valves make them excellent index species.
 
  To see images of non-marine ostracodes in the NANODe (North American Non-Marine Ostracode Database, click here.
 

Subphylum Chelicerata (Cambrian-Recent)
Subclass Eurypterida (Ordovician-Permian)

Image shown with permission from the University of California Museum of Paleontology