| Related Pages:
Glossary of terms
This page last revised 06 August 2009 by S. M. Gon III
The figure below shows the underside of a typical trilobite fossil, with cephalic, thoracic, and pygidial doublure, the rostral plate and associated sutures, and hypostome. The dark gray area represents the hollow interior of the dorsal shell. In this case, the hypostome is separated from the rostral plate, which is called the natant condition. In other species, the hypostome may be connected to the rostral plate (the conterminant condition), separated only by a suture (as in the ventral reconstruction of Olenoides serratus below), or even fused to the rostral plate. These hypostome types are important in trilobite classification.
If you want more detailed definitions of the terms above, I have provided a glossary.
©1999-2006 by S. M. Gon III. Created using Macromedia Freehand
Meanwhile, here is a depiction of the underside of a trilobite, as it might have looked like when it was alive:
Ventral reconstruction of Olenoides serratus
This figure ©2005 by S. M. Gon III
ventral limbs of a trilobite
In this depiction, the limbs and other ventral parts of the Burgess shale trilobite Olenoides serratus are shown. As mentioned above, only very rarely are these structures preserved and fossilized (other than the hypostome). Long, many-segmented antennae emerge from lateral notches of the hypostome (which overlies the mouth), and many pairs of legs, varying very little except in size, proceed from the cephalon to the pygidium (three limb pairs under the cephalon, and one pair for each axial segment in the thorax and pygidium). This primitive lack of specialization is one of the features of trilobite limbs, shared with many other Paleozoic Arachnomorpha.
The limbs are attached to a sequential set of axial sternites (ventral segments) bearing a thin, uncalcified exoskeleton. Each of the bases of the limbs possess jagged toothlike structures that are thought to have processed food passed between the legs forward to the hypostome and mouth. Such food-processing limb bases are referred to as gnathobases (gnathos = jaw). (see Trilobite Feeding Behavior.
Between the endopods (crawling limbs) and the body are pairs of finely branched feathery structures (typically interpreted as gills), borne on the exopods and here colored red, (see additional detail on trilobite limbs, below). In some trilobites, it is thought that movements of the exopods might have allowed the animal to swim (as similar movements provide swimming locomotion in modern marine and freshwater arthropods). Finally, at the rear of the trilobite, two antenna-like cerci (sense organs) are depicted, although Olenoides serratus is the only species among all trilobites that are known to have borne them.
and antennae of trilobites?
Very rarely, conditions for preservation are so good that these delicate features are preserved. Only about twenty different species of trilobites have been found with preservation of antennae and limbs. To the right is an example of the olenid trilobite Triarthrus eatoni showing preserved antennae, legs, and gill filaments. The image can be found at Per Hansson's Trilobite Gallery, which I encourage you to visit!
Similarly, some of the Burgess Shale trilobites, notably Olenoides serratus, show antennae, limbs, and anal cerci, as in the double specimen below (image via the Smithsonian Institution). Images such as these were used to create the reconstruction of Olenoides above.
The limb details of Olenoides show that it was probably a predator or scavenger, bearing spines and a heavy gnathobase with which to tear at the tissues of its prey.
The limb details of Triarthrus (below) are much more delicate, compared to Olenoides ((left), but there are many similarities in limb structure, notably the number of segments in the endopod (walking leg), and the delicate gill filaments of the exopod. These similarities were once considered an important shared character among trilobites and their relatives, but we now know that many of the Paleozoic arthropods have similar limb structure, and that limb similarity points to a shared primitive condition.
On the other hand, some trilobites have limbs that seem quite dissimilar to the pattern above. The limbs of Ceraurus bear an unusual paddle-like exopod with several distinct segments, while the endopod is remarkably simple and unadorned.
Perhaps the most unusual of known trilobite limbs are those borne by Agnostida. They are so unusual that some scientists cite Agnostine limbs as evidence that they must not be true trilobites!
2003 paper by Nigel Hughes reviews all of the trilobite species with
descriptions of ventral features, presenting a summary table including
the data below. It indicates that the typical trilobite bore a pair of
antennae, then 3 cephalic limbs, followed by trunk (thorax + pygidium)
limbs of variable number, depending on the number of thoracic and
pygidial segments. The typical limb consisted of 6 or 7 podomeres. Olenoides serratus remains the only trilobite with antenniform posterior cerci preserved.
Here is an example specimen of Rhenops cf. anserinus from Hunsruck, Germany, that has been prepared to exposed both dorsal and ventral surfaces.
Limbs and antennae are preserved in pyrite.
Image courtesy of Andreas Ruckert:
At the 2008 Trilobite Conference in Spain, this image of the New York trilobite Triarthrus eatoni won wide acclaim as the best preserved cephalic ventral features of a trilobite yet found. The arrangement of the cephalic limbs converging on the hypostome indicate how the gnathobases act as mouthparts processing and manipulating food in the vicinity of the mouth, which underlies the hypostome. Image courtesy of the collection of Dr. Ed Staver:
Bartels, C., D.E.G. Briggs, and G. Brassel. 1998. The Fossils of the Hunsruck slate: Marine life in the Devonian. Cambridge Paleontological Series Number 3. Cambridge University Press, Cambridge.
Bergstrom, J. & G. Brassel 1984. Legs in the trilobite Rhenops from the lower Devonian Hunsruck Shale. Lethaia 17:67-72.
Bruton, D.L. & W. Haas 1999. The anatomy and functional morphology of Phacops (Trilobita) from the Hunsruck Slate (Devonian). Palaeontographica Abteilung A 253:1-75.
Cisne, J.L. 1973. Life history of an Ordovician trilobite Triarthrus eatoni. Ecology 54:135-42.
Dunbar, C.O. 1925. Antennae in Olenellus getzi, n. sp. Amer. J. of Science. Series 5. 9:303-8.
Edgecombe, G.D. & L. Ramskold 1999. Relationships of Cambrian Arachnata and the systematic position of Trilobita. J. Paleontol. 73:263-87.
Hughes, N.C. 2003. Trilobite tagmosis and body patterning from morphological and developmental perspectives. Integr. Comp. Biol. 43:185
Muller, K.J. & D. Wallossek 1987. Morphology, ontogeny, and life habit of Agnostus pisiformis from the Upper Cambrian of Sweden. Fossils and Strata 19:1-124.
Ramskold L. & G.D. Edgecombe 1996. Trilobite appendage structure -- Eoredlichia reconsidered. Alcheringa 20:269-76.
Raymond P.E.1920. The appendages, anatomy, and relationships of trilobites. Memoirs of the Connecticut Academy of Sciences 7:1-169.
Ross, R.J. Jr. 1979. Additional trilobites from the Ordovician of Kentucky. United States Geological Survey Professional Paper 1066-D:1-13.
Shu, D., G. Geyer, L. Chen, and X. Zhang. 1995. Redlichiacean trilobites with preserved soft-parts from the lower Cambrian Chengjiang fauna (South China). Berlingeria, Special Issue 2:203-41.
Stormer, L. 1939. Studies on trilobite morphology, Part I. The thoracic appendages and their phylogenetic significanceNorsk. Geol. Tidssk. 19:143-274.
Stormer, L. 1951. Studies on trilobite morphology, Part III. The ventral cephalic sutures, with remarks on the zoological position of the trilobites. Norsk. Geol. Tidssk. 29:108-58.
Stürmer, W. & Bergström, J. 1973: New
discoveries on trilobites by X-rays. Paläontologische Zeitschrift
Walcott, C.D. 1918. Cambrian geology and paleontology IV. No. 4. Appendages of trilobites. Smithsonian Miscellaneous Collections 67:115-216.
Walcott, C.D.1921. Cambrian geology and paleontology IV. Notes on structure of Neolenus. Smithsonian Miscellaneous Collections 67:365-456.
Whittington, H.B. 1975. Trilobites with appendages from the Middle Cambrian Burgess Shale, British Columbia. Fossils and Strata 4:97-136.
Whittington, H.B. 1980. Exoskeleton, moult stage, appendage morphology, and habits of the Middle Cambrian trilobite Olenoides serratus. palaeontology 23:171-204.
Whittington, H.B. 1993. Anatomy of the Ordovician trilobite Placoparia. Phil. Trans. R. Soc. London. series B 339:109-18.
Whittington, H.B. & J.E. Almond 1987. Appendages and habits of the Upper Ordovician trilobite Triarthrus eatoni. Phil. Trans. R. Soc. London. series B 317:1-46.
Walking Trilobite animation ©2000 by S. M. Gon III