ARTHROPODA

From Eldredge (1991)


INTRODUCTION

Arthropods are the most diverse phylum today and probably also in the geologic past. They are a highly specialized group which are characterized by their bilaterally symmetrical body, paired appendages and a chitinous (calcite in some groups) exoskeleton. As the exoskeleton once produced remains inert, arthropods must periodically shed their exoskeleton during molting and re-precipitate a larger one in order to accommodate their larger size.

Many arthropods may be familiar to you such as crabs, lobsters, barnacles, and insects, yet the fossil record of the phylum is dominated by few groups, particularly the trilobites, and to a lesser extent the eurypterids, and ostracodes. The trilobites in particular are unparalleled for their biostratigraphic utility for Cambrian and Ordovician and to a lesser extent Devonian sediments. Conversely, the eurypterids and ostracodes are more useful in determining ancient environments at least during mid to latter Paleozoic times.


CLASSIFICATION & GEOLOGIC RANGES

Phylum Arthropoda (Precambrian-Recent)

Superclass Trilobitomorpha (Cambrian-Permian)

Class Trilobita (Cambrian-Permian)

Order Polymerida (Cambrian-Permain)

Order Agnostida (Cambrian-Ordovician)

Superclass Crustacea (?Precamb., Cambrian-Recent)

Class Ostracoda (Cambrian-Recent)

Superclass Chelicerata (Cambrian-Recent)

Class Merostomata (Cambrian-Recent)

Order Xiphosurida (Cambrian-Recent)

Order Eurypterida (Ordovician-Permain)

 


TRILOBITOMORPHS

General Morphology

Like other arthropods, trilobitomorphs are characterized by numerous jointed and paired appendages. The calcitic and/or chitinous exoskeleton of trilobites consists of three lobes: a central axial lobe, and two lateral pleural lobes (see Figure 1 below). As shown in Figure 1, the exoskeleton of trilobites can be divided lengthwise into three regions: a fused head segment called a cephalon; a fused tail segment called a pygidium which sometimes bears spines, but is normally completely fused with a smooth margin; and a mid region or thorax consisting of numerous segments. Re-examine the previous images and be sure that you can recognize these morphologic features on a single specimen.

The cephalon also contains several unique features. The glabella is the extension of the axial lobe into the cephalic region and is often ornamented with furrows and ridges. Lateral to the glabella are the eyes which can be either of compound or single lens type. Forward of the glabella is the frontal area, a relatively flat region which sometimes bears pits. The terminal margin of the cephalic segment may be ornamented with genal spines.


Figure 1 - General Trilobite Morphology

Modified from McRoberts (1998)


During molting, many trilobites split their exoskeleton along facial sutures which separates the fixed cheek from the free cheek. Depending on their position relative to the genal angle, these sutures can be of four types: (see Figure 2 below) opisthoparian terminating behind the genal angle, proparian terminating forward of the genal angle, gonatoparian dissecting the genal angle, and marginal along the cephalic margin.


Figure 2 - Trilobite Facial Suture Types

Modified from McRoberts (1998)


Paleoecology and Life Habits

Before any attempt at a paleoecologic or life-habit interpretation is made one must first recognize the nature of the fossil sample. As mentioned earlier, all arthropods periodically molt their exoskeleton to accommodate for larger size; as such, one individual may produce many molts or instars (or sometimes called exuvea). To determine if a trilobite fossil is a molt or body fossil see if the suture is split and the free cheeks are missing. If so, the fossil may be interpreted as a molt. If the suture is intact and there is no evidence of splitting, the fossil may be interpreted as a body fossil. Examine this image and see if you can identify it as a molt or a body fossil.

Trilobites are very common in marine limestones and shales of the early Paleozoic, especially from the Cambrian Period. Most trilobites were epifaunal crawlers. Although they occupy a wide variety of exclusively marine habitats, specific life habits are difficult to discern by morphology alone. Nonetheless, several aspects of trilobite morphology can indeed provide some clue as to the life habit or activity. Examples include the elongated cephalic shield of the example which may have aided in ploughing through sediments. Although most trilobites are considered to have been benthic, the small size and non-descript morphology of agnostid trilobites suggests that these (along with some others) may have been nektonic or nekto-benthic. Enrolling of trilobites may certainly have been a defensive mechanism.


Taxonomy

According to some texts, trilobites are considered to have phylum status and are divided into eight Orders. A less radical classification which is used here (and in other texts) treats trilobites as a Superclass or Class with two orders: the Polymerida and the Agnostida. The Polymerida are by far the most diverse of the two in regards to species diversity and also morphologic and ecologic types. The Polymerids can be identified by their larger size, a well defined cephalic region with eyes and facial sutures, and a large number of thoraxic segments. An easier way to identify Polymerids is by default; if its not a agnostid (easy to identify) then it's a Polymerid. All of the images you have seen thus far are Polymerids, here is another example from this diverse group.

Agnostid trilobites are easily recognizable by their small size, few thoraxic segments (usually around two), and a cephalon without eyes which is superficially similar in morphology to the pygidium. Furthermore, agnostids lack facial sutures.


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CRUSTACEA & CHELICERATA


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