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©1999 - 2008
by S. M. Gon III

Trilobite Enrollment
last revised 13 December 2008 by S.M. Gon III

An enrolled Phacops rana specimen What is enrollment?
Most trilobites could enroll into a defensive ball or capsule, via the flexible articulation of the thoracic segments, bringing the cephalon and pygidium together in a protective closed capsule that shielded the antennae, limbs, and soft ventral surface. While in that enrolled state, the trilobite could watch and wait until conditions were safer (as in the example specimen of Phacops to the left). Some groups of trilobites (e.g. the Phacopina) developed specialized morphological features that aided enrollment, called coaptative structures. These were complementary morphological features that allowed close interlocking of opposing surfaces (coaptation). The cephalon and pygidium of enrolled trilobites often have similar shapes that allow a tight match, even to the point of special notches that fit the edges of enrolled thoracic segments and the pygidial border (see below).
animation of Acaste enrolling
This drawing ©1999 - 2007 by S. M. Gon III
How did trilobites enroll?
In general, trilobites enrolled by contracting internal muscles, bending the flexible integument (shell) between each of the rigid thoracic segments so that the cephalon and pygidium were brought together, and the thoracic pleurae slid into an overlapping radial pattern. Some modern arthropods, such as crustacean isopods, are similarly able to enroll into very compact, sphereical capsules that are resistant to their typical enemies. The animated example above shows the progressive enrollment of a phacopid trilobite: Acaste downingiae. Notice how the limbs and antennae are tucked in to fit within the enrolled exoskeleton, and how in the fully enrolled state, the pygidium and several thoracic pleurae contact the cephalon and no limbs or ventral surfaces are exposed at all. Where the pygidium and thoracic pleurae make contact with the cephalon, there is sometimes a specialized coaptative structure, called the vincular furrow (from the latin vinculus, a locking or binding device), which matches the shape of the pygidial margin and the ends of the thoracic pleurae (see below).

frontal lobe >
of glabella 

vincular >

vincular >

Vincular furrow and notches
Here is the underside of the cephalon of the phacopine trilobite Paciphacops. When enrolled, it would appear rather similar to the enrolled Phacops at the top of this page. The dark grey area is the large concavity of the cephalon, where anterior organs and limbs would be. The lighter grey narrow arch-shaped feature is the vincular furrow, and the vincular notches are the rope-like scalloped lobes arranged at the bottom left and right of the vincular furrow. The furrow accomodates the edge of the pygidium on enrollment, and the notches accomodate the rounded ends of the thoracic pleural segments, forming a near-perfect, tight fit (coaptation). One rounded thoracic pleural termination would fit into each of the vincular notches (see the animated enrollment of Acaste above)

More enrollment examples
Genal and pygidial spines are often designed to offer extra protection when the trilobite is in an enrolled state. Below are the top views of two extended and enrolled phacopine trilobites. Note how the cephalic and pygidial spines offer defense when the animal is in the enrolled state.

bite my spike! I dare you! a bristly beard of spines Want to eat a spine sandwich?
When Dalmanites (above) enrolled (right), an unwieldy triangle of spines was created to present to would-be predators. When Comura (above) enrolled (right), the result was a very discouraging fan of spines.

Types of enrollment: 
Both the 1959 and 1997 Treatises devoted discussions to enrollment and coaptative structures of  trilobites, noting that by virtue of articulation of the thoracic segments, most if not all trilobites could curl the exoskeleton to maximize protection of the vulnerable ventral surfaces and limbs. It is interesting that the most primitive trilobites (e.g., most Olenelloidea) did not have well-developed coaptative structures and could not form closed enrolled capsules. Once enrollment was well established during the Cambrian, post-Cambrian evolution of trilobite exoskeletons seemed restricted by the need to maintain effective enrollment. The 1959 Treatise and Bergstrom's 1973 review of enrollment described some general types of enrollment. These go as far back as Barrande (1852) in which three main types of complete enrollment were named: sphaeroidal, double, and discoidal.

Sphaeroidal enrollment occurs when all of the thoracic segments participate more of less equally in curling the body of the trilobite onto itself. This was described as the most common type of enrollment, especially among isopygous and macropygous trilobites. Agnostida, isopygous trilobites with only two or three segments, nonetheless bring the cephalon and pygidium tightly together, and so qualify as sphaeroidal enrollers. Similarly, enrolling trilobites with relative few segments, such as the Asaphoidea (ca 8 segments) and Phacopoidea (ca 10 segments) generally involve all segments more or less equally in enrollment, and are considered sphaeroidal enrollers (see example Nyterops, right). In fact, the stabilization of segment number in these clades may be due to the strong adaptive advantage of effective enrollment. Variations among sphaeroidal enrolling trilobites include ones in which either the cephalon or the pygidium extend further than the other when enrollment is complete (e.g., Pseudomegalaspis, below).  sphaeroidal
discoidal Double enrollment occurs when the pygidium and hindmost thoracic segments are hidden under the front edge of the cephalon. Seen in primitive Cambrian micropygous trilobites such as Ellipsocephalus, Bergstrom (1973) referred to this kind of enrollment as spiral enrollment.

Discoidal enrollment involves flexing of only the anterior portion of the thorax, while the rest of the thorax and pygidium behaves like a closing lid. This type of enrollment is seen in micropygous trilobites with cephalic development, such as harpetids (example left) and trinucleioids, and was referred to as basket & lid enrollment by Bergstrom (1973).

Incomplete enrollment occurs when the thoracic pleurae do not completely enclose the ventral surfaces at full enrollment, leaving a ring of pleural spines projecting around a lateral gap. This is the case not only with primtive spinose trilobites such as olenelloids, and paradoxodoids, but is seen in more advanced trilobites such as Selenopeltis and some cheirurids. Bergstrom referred to this as cylindrical enrollment.

Some trilobite workers, such as Bergstrom (1973), suggested that the different kinds of enrollment might be useful in classification of trilobites. However, there is a great deal of variability in trilobite enrollment, and other workers suggest that there is so much inconsistency that the utility of enrollment patterns among trilobites for higher level classification is relatively low, or so tied to other obvious morphological features as to be redundant. By the 1997 Treatise, the general view was that the basic enrollment types described were of limited use, and that enrollment was "not a character to be considered fundamental in classification." 

Although of uncertain value for trilobite classification, enrollment remains a fascinating feature of trilobite morphology and worth understanding. Here are a few more images of the enrolled form of various species of trilobites. The different types of enrollment have been labeled, although they are not consistently used. 

Notice how in the case of Ellipsocephalus and Harpes, there is not an exact fit between the cephalon and the enrolled thorax and pygidium. Faint lines show how the thorax and pygidium is tucked within the concavity of the cephalon in those cases. In Pseudomegalaspis, there is an "overbite" that suggests that a portion of the pygidial venter is exposed, but in this case, a very wide pygidial doublure serves as a shield, and there are actually no exposed limbs or soft ventral surface. Finally, below are two images of enrolled trilobites, demonstrating the elegant beauty of the protective capsule formed by enrollment.

enrolled illaenid

A beautiful example of enrollment in the trilobite
Pliomera, family Cheiruridae, Order Phacopida 

Enrolled Illaenus bayfieldi, family Illaenidae, Order Corynexochida

Selected references:

Barrande, J. 1852. System Silurien du Centre de la Boheme. I. Recherches Paleontologiques, vol. 1 (Crustaces: Trilobites) Prague & Paris. xxx + 935 pp., 51 pl.

Bergstrom, J. 1973. Organization, life, and systematics of trilobites. Fossils and Strata 2:1-69.

Clarkson, E.N.K., & H.B. Whittington. 1997. Enrollment and coaptative structures. pp. 67-74 in: R.L. Kaesler, ed. Treatise on Invertebrate Paleontology. Part O: Arthropoda 1, Trilobita, Revised. Lawrence, Kansas.

Harrington, H.J. 1959. Enrollment. pp. O102-O107 in: R.C. Moore, ed. Treatise on Invertebrate Paleontology. Part O:  Arthropoda 1. Lawrence, Kansas.

stroll with me among the topics below...
Walking Trilobite animation ©2000 by S. M. Gon III