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SUBPHYLUM TRILOBITOMORPHA

INTRODUCTION TO THE TRILOBITOMORPHA

Trilobitomorpha (TRI-lo-bi-to-MORF-a) is formed from three roots that mean three lobed forms (three -tri (L.); lobe -lobus (L.); and form is from the Greek word morphi (μορφή)]. The reference is to the three longitudinal lobes that run the length of the body.

The Trilobitomorpha has taxa that were very distinctive and successful in the early part of the Paleozoic Era (especially the Cambrian and Ordovician Periods). They suffered periodic extinctions through the Paleozoic Era and finally disappeared at the great mass extinction event at the end of the Permian Period. The relationship of the trilobites to the other arthropods has been up in the air. They had a distinctive body plan of three tagmata (cephalon, thorax, and pygidium). Also, they had a raised axial lobe flanked by two lateral lobes, a three-lobed structure that provides the name, trilobite (Figures A-D). Because they were among the dominant early animal groups, workers have assumed they trilobites were primitive within the arthropod clade. Thus, they were usually allied with the crustaceans, which also were marine, had biramous walking legs, and a chitinous exoskeleton reinforced with calcium carbonate. The similarity to Limulus (Merostomatida; Cheliceriformes) was accepted as an example of convergent evolution. However, work summarized by Fortey (2001) suggests that the trilobites might best be considered as a class within the Subphylum Cheliceriformes, in which case they would represent about the most derived group of arthropods.

A. Pagetia, a taxon with few segments and relatively large cephalon and pygidium.

B. Cambropallas, a taxon of large species, each with a very small pygidium.

C. Elrathia, a taxon of small species with well-developed cephalon, pygidium, and many body segments.

D. Phacops, a taxon of large-eyed animals which have the ability to roll up.

Images taken from:
A-D: http://www.trilobites.info/

SYNOPTIC DESCRIPTION OF THE SUBPHYLUM TRILOBITOMORPHA

The following information came from Margulis and Schwartz (1998), Brusca and Brusca (2003), Forty (1997), Gon (2003), Hickman (1973), Nielsen (2001), Storer and Usinger (1965), and Tudge (2000).

I. SYNONYMS: trilobites.

II. NUMBER: ~4,000 species known, all extinct.

III. SUBPHYLUM CHARACTERISTICS:

A. Structure

Symmetry: Bilateral; segmented body regions organized into cephalon (head), thorax and pygidium.

Body Cavity: Presumably, they were like other arthropods and had a true coelom that was reduced and absent in adults. Haemocoel the only body cavity.

Body Covering: Covered by chitinous exoskeleton, reinforced by calcium carbonate.

Support: Hardened exoskeleton.

Digestive System: Food tube simple. Mouth at anterioventral end. Likely they tore apart their food by spiny processes on their thoracic legs. The pieces, thus rendered, were passed forward to the mouth for ingestion.

Circulatory System: Likely, they had an open system. Haemocoel of blood sinuses with a dorsal circulatory vessel (heart).

Locomotion: A pair of biramous walking legs per thoracic segment.

Excretory System: Not known.

Nervous System: Probably with circumesophagial ring and ventral cords ganglionated at each segment. The cephalon had a pair of sensory antennae. Compound eyes of unusual structure. Lenses of calcium carbonate crystals. At least one group had sensory cerci.

Endocrine System: Not known.

Respiratory System The exopodites of the biramous walking leg likely functioned as gills.

B. Reproduction:

Reproductive System: Likely they were oviparous.

Development: The development of trilobites occurred through successive molts in three periods: Protaspid Period (in this period the larvae have no segmentation), Meraspid Period (in this period the larvae have segments and add them through successive molts until they achieve the adult number), Holaspid Period (the stage at which the animal has achieved the adult number of segments and molts as it increases in size).

C. Ecology: Mostly free-living scavenger-predators, benthic marine (though a few seem to have been adapted to swimming in the open water).

HIERARCHICAL CLASSIFICATION OF THE TRILOBITOMORPHA

This taxonomic system comes from Fortey (1997) and has a single class (Class Trilobita) with nine orders.

CLASS TRILOBITA

Olenellus, Bristolia, Gabriellus, Peachella, Callavia, Fallotaspis, Balcoracania, Richterops, Emuella, Redlichia, Eops, Paradoxides, Anopolenus.

Agnostus, Pagetia, Condylopyge, Ekwipagetia, Peronopsis, Lejopyge, Pleuroctenium, Dawsonia, Tsunydiscus.

Polypleuaspis, Dorypyge, Fieldaspis, Bumastus, Scutellum, Mansuyia, Dysplanus, Bathyuriscus, Zacanthroides, Lancastria, Oryctocephalus, Thoracocare, Kolihapeltis, Harpillaenus, Szechuanella.

Arctinurus, Dicranopeltis, Kettneraspis, Damesella, Selenopeltis, Boedaspis, Miraspis, Dicranurus, Stelckaspis, Drepanura, Palaeadotes.

Modocia, Elrathia, Ellipsocephalus, Tricrepicephalus, Olenus, Densonella, Asaphiscus, Conocoryphe, Cedaria, Peltura, Triarthrus, Leptoplastus, Balnibarbi.

Harpes, Entomaspis, Dolichoharpes, Harpides, Eoharpes.

Phacops, Calymene, Cheiurus, Reedops, Dalmanites, Rhenops, Comura, Trimerus, Huntonia, Acastes, Comura, Philonyx, Odontochile, Odontocephalus, Bavarilla, Parahomalonotus, Placoparia, Deiphon.

Isotelus, Housia, Asaphus, Cyclopyge, Cryptolithus, Dikelokephalus, Remopleurides, Lachnostoma, Pterocephalia, Ogygiocaris, Dikelokephalina, Saukia, Opipeuter, Robergia, Novakella, Nileus, Pricyclopyge, Neoasaphus, Ogygiocaris, Labiostria, Ampyx, Taklamakania, Glyphaspis, Seleneceme.

Comuproteus, Cyphoproteus, Aulacopleura, Goniotelus, Phillipsia, Otarion, Bathyurus, Gerastos, Phaetonellus, Telephinus, Carolinites, Uromystrum, Cordania, Brachymetopus.

This page is maintained by Jack R. Holt. Last modified: 01/07/08.