Related Pages:
Dorsal Morphology
Major Features
Internal Anatomy
Pygidium Size
Hypostome Types
Facial Sutures
Glossary of terms

Trilobite Ventral Structures
This page last revised 06 August 2009 by S. M. Gon III

Hey! Watch what you're touching there, buster! Unlike the thicker dorsal shell of a trilobite, many of the ventral (underside) features, including limbs and antennae, usually are not preserved. The ventral portions that are typically preserved include the doublure (a ventral extension of the dorsal exoskeleton), a special part of the doublure, typically separated by sutures at the anterior of the cephalon, called the rostral plate, and a hard mouthpart called the hypostome, that typically underlies the glabella.

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.

Sometimes I feel a mere shell of my former self...
©1999-2006 by S. M. Gon III. Created using Macromedia Freehand
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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
Ventral reconstruction of Olenoides serratus
This figure ©2005 by S. M. Gon III

A recontruction of the 
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.

How do we know about the limbs
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.

Rare specimen of Olenoides with limbs preserved

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. 

Shown below is an excellent ventral preparation of the Russian trilobite Asaphus platyurus from the Saint-Petersburg Paleontological Laboratory. Notice the very wide pygidial doublure, often seen in Asaphoidea. 

Do all Russian trilobites speak with forked hypostome?

Some think Triarthrus could use its long gills to swim

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!

A 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.

Species Age Locality Cephalic limbs Thoracic limbs Pygidial limbs Podomeres Source
Eoredlichia internedius E. Cambrian Chengjiang 3? n/a n/a ?7 Shu et al 1995; Ramskold & Edgecombe 1996
Yunnanocephalus yunnanensis E. Cambrian Chengjiang 4? n/a n/a n/a Shu et al 1995
Olenellus getzi E. Cambrian  Lancaster, PA n/a n/a n/a n/a Dunbar 1925
Olenoides serratus M. Cambrian Burgess Shale 3 7 <6 6 Whittington 1975, '80
Kootenia burgessensis M. Cambrian Burgess Shale n/a n/a n/a n/a Walcott 1918; Raymond 1920
Elrathina cordillerae M. Cambrian Burgess Shale n/a n/a n/a n/a Walcott 1912, '18; Raymond 1920
Elrathia permulta M. Cambrian Burgess Shale n/a n/a n/a n/a Walcott 1918; Raymond 1920
Agnostus pisiformis L. Cambrian Orsten, Sweden 3 2 3 7 Muller & Walossek 1987
Placoparia cambriensis M. Ordovician S. Wales 3/3.5 n/a n/a n/a Whittington 1993; Edgecombe & Ramskold 1999
Isotelus latus L. Ordovician   . n/a ?8 n/a n/a Raymond 1920
Isotelus maximus L. Ordovician   . n/a ?8 ?16 n/a Raymond 1920
Triarthrus eatoni L. Ordovician Beecher Trilobite Bed 3/3.5 14 >10 6 Raymond 1920; Cisne 1973; Whittington & Almond 1987; Edgecombe & Ramskold 1999
Cryptolithus tesselatus M.-L. Ordovician   . n/a ?5 >10 ?7 Raymond 1920; Stormer 1939
Primaspis trentonensis M.-L. Ordovician   . n/a ?10 n/a n/a Raymond 1920; Ross 1979
Primaspis sp. L. Ordovician   . n/a >8 n/a n/a Ross 1979
Ceraurus pleurexanthemus L. Ordovician Walcott-Rust 4 ?11 3 n/a Walcott 1918, '21; Raymond 1920; Stormer 1939, '51
Flexicalymene senaria L. Ordovician Walcott-Rust n/a ?13 >2 n/a Walcott 1918, '21; Raymond 1920
Chotecops ferdinandi E. Devonian Hunsruck 3 11 >12 6 Sturmer & Bergstrom 1973; Bruton & Haas 1999
Asteropyge sp. E. Devonian Hunsruck 3 ?11 >?4 n/a Sturmer & Bergstrom 1973
Rhenops cf. anserinus E. Devonian Hunsruck 3/3.5 11 >6 7 Bergstrom & Brassel 1984; Bartels et al 1998; Edgecombe & Ramskold 1999

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:

Sources cited:

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 eatoniEcology 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 47, 104–141.

Walcott, C.D.1912. Cambrian geology and paleontology II. No. 6. Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata. Smithsonian Miscellaneous Collections 57:145-228.

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.

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Walking Trilobite animation ©2000 by S. M. Gon III