Isotelus, like all
asaphid trilobites, did not survive past the
Ordovician-Silurian mass extinction event. Asaphidae, like all other derived
asaphide trilobite families, are inferred to have a unique and lengthy planktonic larval phase, only becoming benthic like adults after metamorphosis. The first
instar larvae of
Isotelus for instance were just 0.6 mm long, becoming benthic and adult-like at 8–10 mm long. The larvae of Isotelus had a globular shape, with a pair of long spines on the anterior and ventral extremities of the exoskeleton; attached was a large hypostome that mostly enclosed the dorsal shell, which itself was covered in a number of spines. The exoskeleton consisted of a single piece representing both the proto-cephalon and proto-pygidium, with sutures separating the already developed
free cheeks. There were a small number of
ocelli eyes on the free cheeks. In addition to multiple stages of the protaspid phase, there was likely a stage prior to protaspid where the trilobite was soft-bodied, and thus left no obvious fossil record. As there is a wide diversity of adult morphology in the aforementioned trilobite families, this tiny planktonic larval phase that they share may be the link to their extinction. Most trilobites with this life history strategy lived in warm, low latitude waters, in which planktonic, non-adult like larvae may be ideal at surviving in.
Isotelus specimens with pathologies are rare, but some have been reported, including a specimen of
Isotelus iowensis with an injury to the border of the cephalon. This injury was relatively small, and probably represents a failed predation attempt.
Feeding The large, forked hypostome of Isotelus was rigidly attached to the doublure of the hypostome, and displays no adaptations suitable for filter feeding. The dorsal surface was marked with wavy terraces, but the inner, ventral surface was covered with much finer, unidirectional terraces, which have been theorized to function as a grinding surface. This theory suggested the presence of a differentiated pair of legs which were modified to grind prey against the hypostome, which would break the trend among known trilobites of the limbs being all undifferentiated. Fossil evidence for this limb pair (apparently the second), supposed to be smaller and directed inwardly towards the hypostome, awaits publication. Furthermore, an exceptional, long specimen of
Rusophycus carleyi, attributable to
Isotelus generally, was discovered in the Ordovician strata of
Clermont County, Ohio. This specimen is more detailed than typical
Rusophycus, preserving the entire outline of the producer including the large genal spines, confirming its identity as
Isotelus. It also includes a large, secondary trace of a worm burrow, passing beneath the trilobite and stopping beneath the head, the burrowing activity apparently halted by the trilobite suddenly digging in with all of its limbs, finding the worm, and devouring it before moving on.
Burrowing and terrace structures Although a cluster of small shallow pits in the holotype of
I. rex have been suggested to be caused by soft-bodied
epibionts in life, even the largest specimens of
Isotelus are rarely associated with any kind of
symbiotic encrusting which is commonly found in large modern marine arthropods. This has been suggested as being due to either a low number of encrusting organisms in the environment, cuticular anti-fouling properties, or most likely, a semi-
infaunal (below the sediment) mode of life, precluding extensive colonization by any encrusting organism.
Isotelus had a large, shovel-like cephalon (comparable to the more developed shovel morphology of
Trimerus), and the suggestion that
Isotelus was a burrower is supported by numerous trace fossil specimens, including large traces ascribed to
I. rex. It is suggested that
Isotelus often buried itself in sediment, with only its eyes and portions of the dorsal exoskeleton exposed. It is suspected that such a life style made it easy for them to become fossilized by allowing rapid burial and preventing the carcasses from becoming disarticulated. As the trilobite grew, the terraces were copied onto the new exoskeleton, with new terraces occasionally added between the old ones to maintain a constant distance between the ridges throughout ontogeny. The function of the terrace ridges is not confirmed, but they are theorized to have played a role in either controlling friction and sediment guiding during burrowing, or more likely, a
mechanoreceptor function. The canals and pits are believed to have originally house tiny sensory hairs called setae (more specifically, the smaller setae are referred to as microtrichs, and the larger as macrotrichs), which would have (in the case of the pits along the scarp) been angled in the direction of the terraces. Like some modern crustaceans (such as
Procambarus,
Homarus, and
Asellus), these sensory hairs, angled by the terrace ridge, could have served as current detectors – as water ran over the hair and deflected it one way or another, the trilobite could sense which way the water current was moving over its body. The accessory pits and associated macrotrich setae could have provided additional tactile information.
Distribution Isotelus is known from across North America, in the
United States and
Canada.
Isotelus is usually associated with typical Ordovician fauna, including
brachiopods, various
crinoids, nautiloids, other trilobites, the tubeworm
Cornulites, and soft-bodied annelids known from trace fossil evidence. It is usually found in warm shallow seas, in shelf-environments. Specimens of the species
I. rex were found in the
Churchill River Group, in sediments that were once a warm, rocky, shallow subtidal zone along an Ordovician shoreline. During the Late Ordovician, this area would have been at the equator. == Species ==