DESCRIPTION OF THE SUBPHYLUM TRILOBITOMORPHA (MOORE 1959)

EUKARYA> UNIKONTA> OPISTHOKONTA> ANIMALIA> BILATERIA> PROTOSTOMATA> ECDYSOZOA> PANARTHROPODA> ARTHROPODA> 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. Walch (1771) coined the term Trilobita. Trilobitomorpha was a taxon created by Moore (1959) to make a subphylum rank equal to the arachnomorphs.

INTRODUCTION TO THE TRILOBITOMORPHA 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 in 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 1-4). Because they were among the dominant early animal groups, workers have assumed the 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 (see Figure 5 below and the Major Clades of the Panarthropods on the Arthropod Page).

FIGURE 1. Pagetia, a taxon with few segments and relatively large cephalon and pygidium. Image from http://www.trilobites.info/	FIGURE 2. Cambropallas, a taxon of large species, each with a very small pygidium. Image from http://www.trilobites.info/	FIGURE 3. Elrathia, a taxon of small species with well-developed cephalon, pygidium, and many body segments. Image from http://www.trilobites.info/	FIGURE 4. Phacops, a taxon of large-eyed animals which have the ability to roll up. Image from http://www.trilobites.info/

	Am = Arachnomorpha Ch = Cheliceriformes Eu = Euchelicerata
FIGURE 5. MAJOR CLADES OF THE CHELICERIFORMES AND THE ASSOCIATED TRILOBITES. This is a portion of the Figure 2 on the Arthropoda page and the clade designations should refer back to that cladogram. This cladogram reflects the Arachnomorpha hypothesis in which the trilobites are sisters to the chelicerates.

LITERATURE CITED Averof, M. and M. Akam. 1995. Insect-crustacean relationships: insights from comparative developmental and molecular studies. Phil. Trans. R. Soc. London. B. 347: 293-303. Ax, P. 2000. Multicellular Animals II. Springer Verlag. Berlin. Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass. Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago. Budd, G. E. 1998. Arthropod body plan evolution in the Cambrian with an example from anomalocaridid muscle. Lethaia. 31: 197-210. Budd, G. E. 2001. Tardigrades as ‘Stem-Group Arthropods’: The evidence from the Cambrian fauna. Zool. Anz. 240: 265-279. Conway Morris, S. (1998). The crucible of creation: the Burgess Shale and the rise of animals. Oxford [Oxfordshire]: Oxford University Press. pp. 56–9. Dunn, C. W., A. Hejnol, D. Q. Matus, K. Pang, W. E. Browne, S. A. Smith, E. Seaver, G. W. Rouse, M. Obst, G. D. Edgecombe, M. V. Sørensen, S. H. D. Haddock, A. Schmidt-Rhaesa, A. Okusu, R. M. Kristensen, W. C. Wheeler, M. Q. Martindale, and G. Giribet. 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature. 452: 745-749. Fortey, R.A. 1997. Phylogenetic Concepts in the History of Trilobite Classification. Second International Trilobite Conference, Brock University, St. Catharines, Ontario. Fortey, R. A. 2001. Trilobite Systematics: The last 75 years. Journal of Paleontology. 75(6): 1141-1151. Garey, J. R. 2001. Ecdysozoa: The relationship between Cycloneuralia and Panarthropoda. Zoologischer Anzeiger 240: 321-330. Giribet, G., G. D. Edgecombe, J. M. Carpenter, C. A. D’Haese, and W. C. Wheeler. 2004. Is Ellipura monophyletic? A combined analysis of basal hexapod relationships with emphasis on the origin of insects. Organisms, Diversity and Evolution. 4: 319-340. Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis. Ivantsov, A. Yu. 2004. New Proarticulata from the Vendian of the Arkhangel’sk Region. Paleontological Journal. 38(3): 247-253. Lavrov, D. V., W. M. Brown, and J. L. Boore. 2004. Phylogenetic position of the Pentastomida and (pan)crustacean relationships. Proceedings of the Royal Society of London. Series B. 271: 537-544. Mallatt, J. M., J. R. Garey, and J. W. Shultz. 2003. Ecdysozoan phylogeny and Baysean inference: first use of nearly complete 28S and 18S rRNA gene sequences to classify the arthropods and their kin. Molecular Phylogenetics and Evolution. 31: 178-191. Manton, S. F. 1977. The arthropod habits, functional morphology, and evolution. Clarendon Press. Oxford. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. Mayer, G. 2006. Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods? Arthropod Structure and Development. 35: 231-245. Mayer, G. and P. M. Whittington. 2009. Velvet worm development links myriapods with chelicerates. Moore, R. C. ed. 1959. Arthropoda 1. Part O. Treatise on Invertebrate Paleontology. The Geological Society of America and University of Kansas Press. Lawrence, Kansas. pp. 1-560. Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. 2nd Edition. Oxford University Press. Oxford. Patel, N. H., E. Martin-Blanco, K. G. Coleman, S. J. Poole, M. C. Ellis, T. B. Kornberg, and C. S. Goodman. 1989. Expression of engrailed proteins in arthropods, annelids, and chordates. Cell. 58: 955-968. Pechenik, J. A. 2005. Biology of the Invertebrates. McGraw-Hill. New York. Regier, J. C., J. W. Shultz, and R. E. Kambic. 2005. Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic. Proceedings of the Royal Society of London. Series B. 272: 395-401. Reiger, J. C., J. W. Schultz, A. R. D. Ganley, A. Hussey, D. Shi, B. Ball, A. Zwick, J. E. Stajich, M. P. Cummings, J. W. Martin, and C. W. Cunningham. 2008)Resolving arthropod phylogeny: exploring phylogenetic signal within 41 kb of protein-coding nuclear gene sequence. Syste. Biol 57(6): 920-938. Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. 6th edition. Saunders. Ft Worth, TX. Ruppert, E. E., R. S. Fox, and R. D. Barnes. 2004. Invertebrate Zoology: A Functional Evolutionary Approach. Seventh Edition. Thomson, Brooks/Cole. New York. pp. 1-963. Strausfeld, N. J., C. M. Strausfeld, R. Loesel, D. Rowell, and S. Stowe. 2006. Arthropod phylogeny: onychophoran brain organization suggests an archaic relationship with a chelicerate stem lineage. Proc. R. Soc. London. B. 273: 1857-1866. Telford, M. J. S. J. Bourlat, A. Economou, D. Papillion, and O. Rota-Stabelli. 2008. The evolution of Ecdysozoa. Phil. Trans. R. Soc. B. 363: 1529-1537. Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York. Waggoner, B. M. 1996. Phylogenetic hypotheses of the relationships of arthropods to Precambrian and Cambrian problematic fossil taxa. Systematic Biology 45(2): 190-222. Walch, J. E. I. 1771. Die Naturgeschichte der Versteinerungen. Dritter Theil. , Paul Jonathan Felstecker. Nuremberg. pp. 1-235. Whittington, H. B. and D. E. G. Briggs. 1985. The largest Cambrian animal, Anomalocaris, Burgess Shale, British Columbia. Phil. Trans. R. Soc. London. B. 309: 569-609. Willmer, P. 1990. Invertebrate relationships, patterns in animal evolution. Cambridge University Press. Cambridge.

By Jack R. Holt and Carlos A. Iudica. Last revised: 02/05/2015