Invertebrate Anatomy OnLine

Elrathia   ©



Copyright 2001 by

Richard Fox

Lander University


            This is one of many exercises available from Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology.   Additional exercises can be accessed by clicking on the link.   A glossary and chapters on supplies and laboratory techniques are also available through this link.   Terminology and phylogeny used in these exercises correspond to usage in the textbook by Ruppert, Fox, and Barnes (2004).   Hyphenated figure callouts refer to figures in the textbook.   Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises.  


Arthropoda P, Trilobitomorpha sP, Ptychopariida O


            Arthropoda, by far the largest and most diverse animal taxon, includes chelicerates, insects, myriapods, and crustaceans as well as many extinct taxa such as Trilobitomorpha.   The segmented body primitively bears a pair of jointed appendages on each segment.   The epidermis secretes a complex cuticular exoskeleton which must be molted to permit increase in size.   Extant arthropods exhibit regional specialization in the structure and function of segments and appendages but the ancestor probably had similar appendages on all segments. The body is typically divided into a head and trunk, of which the trunk is often itself divided into thorax and abdomen.      

            The gut consists of foregut, midgut, and hindgut and extends the length of the body from anterior mouth to posterior anus.   Foregut and hindgut are epidermal invaginations, being derived from the embryonic stomodeum and proctodeum respectively, and are lined by cuticle, as are all epidermal surfaces.   The midgut is endodermal and is responsible for most enzyme secretion, hydrolysis, and absorption.

            The coelom is reduced to small spaces associated with the gonads and kidney.   The functional body cavity is a spacious hemocoel divided by a horizontal diaphragm into a dorsal pericardial sinus and a much larger perivisceral sinus.   Sometimes there is a small ventral perineural sinus surrounding the ventral nerve cord.  

            The hemal system includes a dorsal, contractile, tubular, ostiate heart that pumps blood to the hemocoel.   Excretory organs vary with taxon and include Malpighian tubules, saccate nephridia, and nephrocytes.  Respiratory organs also vary with taxon and include many types of gills, book lungs, and tracheae.   

            The nervous system consists of a dorsal, anterior brain of two or three pairs of ganglia, circumenteric connectives, and a paired ventral nerve cord with segmental ganglia and segmental peripheral nerves.   Various degrees of condensation and cephalization are found in different taxa.

            Development is derived with centrolecithal eggs and superficial cleavage.   There is frequently a larva although development is direct in many. Juveniles pass through a series of instars separated by molts until reaching the adult size and reproductive condition.   At this time molting and growth may cease or continue, depending on taxon.


            The extinct trilobites are among the earliest known arthropods, appearing first as Cambrian fossils and departing forever in the Permian.   Far from being primitive, however, they are already highly derived, specialized arthropods when they make their first appearance and undergo relatively little change over their 300 million year history. These are the best known, to science and the public alike, of fossil invertebrates and are common worldwide.

Although there are eight orders (Fig. 17-7*) and over 15,000 species in 150 families, trilobites exhibit a relatively uniform body plan that includes a heavily calcified dorsal exoskeleton, segmented body with paired, biramous, segmental appendages, and tagmosis consisting of anterior cephalon, middle thorax, and posterior pygidium.   The ventral surface is uncalcified and its details, including appendages, rarely fossilize.   The body is dorsoventrally flattened, or depressed.

The eponymous three lobes lie side by side and consist of a median axial lobe flanked by a pleural lobe on either side. With the exception of the antennae, the apendages are biramous and similar, except for size, over the length of the body. Most are relatively small, 3-10 cm although the full range is 0.5 mm -70 cm.

The appendages have heavy gnathobases whose teeth face, and form, a median food groove.   Presumably, movements of the appendages move food particles anteriorly in the food groove to the posterior-facing mouth where ingestion occurs.   This primitive arthropod feeding mechanism appears in many other taxa including horseshoe crabs.

The lenses of trilobite compound eyes are mineral, being composed of calcite, rather than being organic as are those of all other arthropods. 

A recent interpretation of arthropod phylogeny abandons the name Trilobitomorpha and places trilobites in the taxon Trilobita C.   In this scheme Arthropoda P is divided into the sister taxa Schizoramia sP (with biramous appendages and including Crustacea, Trilobita, Chelicerata) and Atelocerata sP (with uniramous appendages and including Hexapoda and Myriapoda).   In this revision Trilobita and Chelicerata are sister taxa in Arachnomorpha SC, which is itself   the sister taxon of Crustaceomorpha SC within Schizoramia.  

Study Material

            Trilobite fossils and casts of fossils are available from biological supply companies.   This exercise is based on actual fossils but either fossils or casts can be used.   The teaching specimens in the laboratory may belong to several species and in recognition of this the descriptions below are sufficiently general that they can be used with most species.   The ventral surface of most specimens will be embedded in rock or, if exposed, will show little detail and consequently most of your study will be of the dorsal surface.   An excellent and exhaustive treatment of trilobite morphology and identification is provided by Gon (2001).

External Anatomy

Dorsal Surface

Study the dorsal surface of a trilobite using a dissecting microscope whenever the size of the specimen dictates.   The dorsal surface is covered by the calcitic, readily fossilized exoskeleton known as the dorsal shield (Fig. 1).   The delicate, uncalcified, and rarely fossilized ventral membrane extends from the doublure across the ventral surface.

The dorsal shield is conspicuously divided into three side-by-side longitudinal lobes. The central, or median, lobe is the axial lobe and it is the thickest of the three.   In the living trilobite it contained most of the viscera, including, presumably, the gut, heart, and segmentally ganglionated ventral nerve cord (Fig. 17-2A).   On either side of the axial lobe lies a smaller pleural lobe.   Each is separated from the axial lobe by a longitudinal axial furrow.   These three lobes are responsible for the name “ trilobite”.


            In addition to the three longitudinal lobes, the dorsal shield is also divided into three transverse tagma consisting of an anterior cephalon, or head, a middle thorax, and a posterior pygidium.  

The cephalon bears the sense organs and mouth and is usually relatively large.   Internally it housed the brain and anterior gut, including the stomach and digestive ceca (Fig 17-2B).   Dorsally the segmental construction of the cephalon is not obvious (Fig. 1, 17-1).   The trilobed nature of the body, however, is readily apparent on the cephalon. The axial lobe extends anteriorly onto the cephalon where it is known as theglabella and the two pleural lobes are present at the genae, or cheeks.    The glabella may bear transverse or oblique glabellar furrows that mark the divisions between the segments of the head. The heavy margin of the cephalon is its anterior and lateral border.

Each gena usually bears a compound eye although eyes are absent in some taxa.   Each eye is composed of numerous ommatidia whose lenses, or facets, are visible on the surface.   In most trilobites the lenses are numerous (up to 15,000 in each eye) and contiguous so that each is hexagonal in shape.   A common cornea covers al the lenses of the eye.   Such eyes are holochroal.   The schizocroal eyes characteristic of one trilobite order have larger ommatidia whose circular lenses are widely spaced and do not touch (Fig. 17-2B).   Each lens is covered by its own cornea.  What kind of eyes does your specimen have? ____________ Unlike all other arthropods, trilobites lenses are mineral, made of calcite, rather than organic cuticle.   Each lens is a single calcite crystal.

Figure 1. The dorsal surface of the ptychopariid trilobite, Elrathia.  This 26 mm specimen, of unknown provenance, was purchased from a biological supply company. Trilob56L.gif

  Figure 1 

In many taxa each of the two posterior corners of the cephalon bears a genal spine, which may be quite large (Fig. 17-7E,G).   Of considerable importance in trilobite taxonomy and in trilobite biology are thefacial sutures on the genae.   These grooves arise at the anterior margin of the gena and pass posteriorly through the middle of the gena, often separating the eye from the glabella or sometimes bisecting the eye, and then ending on the lateral or posterior margin or the cornerangle of the gena.   The location of the posterior end of the furrow is taxonomically important.   In Fig 1 the suture terminates on the posterior margin of the gena and is opisthoparian.   A suture that ends on the lateral margin is proparian and one that ends at the genal angle is gonatoparian.

The facial sutures are ecdysial lines along which the old exoskeleton splits during ecdysis.  The facial suture divides its gena into a medial fixigena and a lateral librigena (Fig 1).   During ecdysis the fixigena remains fused with the glabella, whereas the librigena is liberated from it.   The combined fixigena and glabella are known as the cranidium.

Look at the ventral surface of the cephalon.   If you could study the ventral membrane of a perfectly preserved cephalon you would see more obvious segmentation that is apparent dorsally (Fig. 17-1B).   The trilobite cephalon is thought to be composed of the acron and four fused segments.   Its appendages are a pair of uniramous antennae and three similar pairs of biramous legs.   Each pair of appendages is associated with a cephalic segment.  

On the ventral surface, corresponding in position to the glabella is a longitudinal ridge covered by a sclerite, the hypostome (Fig 2).   If your specimen has been carefully cleaned, the hypostome may be visible.  The hypostome may homologous to the crustacean labrum.   Unlike the ventral membrane, the hypostome is sclerotized and is often present in fossils.

The mouth opens at the posterior edge of the hypostome and faces posteriorly toward the anterior end of the food groove.   The gut is J-shaped with the esophagus and stomach located in the central lobe (glabella) of the cephalon (Fig 17-2B).   Large digestive ceca extend from the stomach into the genae. The stomach is the anterior-most region of the gut and the esophagus extends posteriorly to connect with the backward-facing mouth (Fig. 17-5B). The gut was a long tube housed in the axial lobe and ending at a posterior ventral anus on the pygidium (Fig 17-2A, 17-3, 17-5B).


            The remainder of the body is the trunk, divided into thorax and pygidium, both of which are conspicuously segmented (Fig 1).   The thorax, which is the middle region of the body, is usually, but not always, much larger than the pygidium.   The thorax may consist of 2-60 independent segments articulated, fore and aft with each other by flexible articular membranes.   Each segment has a central axial ring from which extend, one right and one left, two pleurites.   The combined axial rings of all the segments make up the axial lobe, just as the combined pleurites on each side make up the two pleural lobes.   In some taxa the pleurites bear lateral spines (Fig 17-7A).

The flexibly articulated segments of the thorax allow for enrollment. In this maneuver the animal arches the body dorsally and brings the ventral surfaces of the pygidium and cephalon in contact with each other (Fig 17-5).   This protects the vulnerable ventral membrane and appendages from attack.   Most trilobites were able to enroll but some were better adapted for it than others.


            Like the thorax, the pygidium is composed of appendage-bearing segments but in this case the segments are fused into a rigid platform and are not connected by flexible articular membranes (Fig 1). The pygidium is the posteriormost tagma.   Although fused, the pygidial segments are distinct.   Pygidial segments and their appendages diminish rapidly in size posteriorly.   The axial lobe extends posteriorly onto the pygidium but tapers to a blunt point posteriorly.   The right and left pleural lobes also extend onto the pgidium where they join each other posterior to the end of the axial lobe.    The thickened margin of the pygidium is the pygidial border. The relative size of the pygidium and cephalon varies with taxon and is an important character in trilobite classification and identification (Fig 17-7).

Internal Anatomy

            Little is known of the internal anatomy of these extinct arthropods but some information is available from X-ray studies and the remainder is conjecture based on our knowledge of extant arthropods.  

Ventral Surface

With the exception of the doublure and hypostome, the ventral surface consists of the delicate, weakly calcified ventral membrane which almost never fossilizes.   The segmental appendages are ventral but, like the ventral membrane, are rarely present in fossils.

Along its margins the dorsal shield turns ventrally to form a thick, marginal,   ventral shelf of calcified exoskeleton known as the doublure (Fig 2, 17-1, 17-5C, 17-3).   The doublure typically fossilizes along with the dorsal shield.   If your specimen has been carefully cleaned you may be able to see the doublure and the hypostome but little else on the ventral surface. The doublure consists of cephalic, pleural, and pygidial regions.

Figure 2 Ventral surface of the Elrathia specimen in Fig 1. Trilob57L.gif

  Figure 2

*Hyphenated call-outs, such as this one, refer to figures in Ruppert, Fox, and Barnes (2004).   Call-outs without hyphenation refer to figures embedded in this exercise.


Fortey RA.   1997. Classification, in Kaesler RL (ed.) Treatise on Invertebrate Paleontology, Part O Arthropoda 1, Trilobita, revised. Vol. 1: Introduction, Order Agnostida, Order Redlichiida. Geological Society America & Univ. Kansas, Boulder, CO. 530pp.

Fortey RA.   2000. Trilobite, Eyewitness to evolution.   Vintage Books, New York. 284 pp.

Fortey RA. 2004. The lifestyles of the trilobites. Am. Sci. 92(5):446-453.

 Gon S. 2001. A pictorial guide to the orders of trilobites. Privately published and available for $15 from S. Gon, 1604 Olalahina Pl, Honolulu, HI, 96807. 90 pp.

Gon S. A guide to the orders of trilobites.

Ruppert EE, Fox RS, Barnes RB.   2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.  



1 trilobite, real or cast

1 dissecting microscope


Elrathia fossils are available from Carolina Biological Supply Company