is an extinct genus of large asaphid trilobites from the Middle and Late Ordovician period, fairly common in the northeastern United States into eastern Canada. Isotelus is the state fossil of Ohio, and through multiple specimens from the 1800s into the modern day has held the title of largest trilobite fossil in the world, reaching over long. Isotelus was carnivorous and a burrower which lived in warm shallow seas, feeding on worms and other soft-bodied animals on and below the substrate. As larvae, Isotelus was planktonic, drifting in the water column with a morphology very different from the adult. Their distinctive morphology with a large shovel-like head, prominent eyes, long genal spines, and their enormous size, makes Isotelus one of the most sought after trilobites in North America.
History
By published accounts, fossils of Isotelus were first discovered by John Sherman at Trenton Falls, within the rocks of the Trenton Group, and later described by palaeontologist and zoologist James Ellsworth De Kay in 1824. He named the specimens Isotelus gigas in reference to their large size, and a smaller specimen which he named Isotelus planus, which he considered possibly synonymous with a larger species (a view upheld by Rudkin and Tripp). It was also discovered early on near Dunkinsville, Ohio, where palaeontologist John Locke, as part of a long-running geological survey of the state, discovered a single large fragment of Isotelus, the pygidium. By comparing it with complete specimens of other species he determined this individual was probably around in length. He named the new species Isotelus maximus in reference to its enormous size, noting that locals of the area referred to trilobite fossils as "petrified locusts, butterflies, bugs, and frogs". In the autumn of 1882, professor John Mickleborough was sent a fossil (found by a Mr. D. S. McCord, an Ohioan resident) of I. maximus (which he referred to as Asaphus megistos) that was exposed ventrally, and apparently preserved the legs. The counterpart of the same specimen was discovered a year later and sent as well. This was the first ever definitive fossil to preserve the legs of a trilobite, which despite their extensive fossil record, had otherwise remained enigmatic, and so he described the fossil in 1883. This description was followed up with studies of the fossil by Charles D. Walcott, who revealed further details including possible filaments of the exopodite gills. Numerous other species of Isotelus were discovered in the following century.
For many years, the largest complete specimen of a trilobite found anywhere in the world was a specimen of Isotelus brachycephalus (now generally considered a synonym of I. maximus) unearthed during the construction of an outlet tunnel for the Huffman Dam by workmen (who initially reported it as a fossil turtle), the specimen measuring in length and wide, a long-time display piece of the National Museum of Natural History. Inspired by this large fossil, a group of students local to the Dayton area successfully petitioned the local government to establish Isotelus as the official state fossil of Ohio on June 20, 1985 (later alongside Dunkleosteus). The title of largest complete trilobite in the world would be claimed again by Isotelus with the discovery of a complete specimen of the new species Isotelus rex by a team in Manitoba. Isotelus remains a highly sought-after trilobite among private collectors and museums.
Description
Isotelus is a large trilobite, with I. maximus reaching up to at least in length, and I. rex up to in length. The dorsal shell, like all trilobites, consisted of a cephalon, a multisegmented thorax, and a pygidium. The cephalon was large, rounded, and mostly smooth (some small pores are found on the cephalon, particularly the free cheeks, or 'librigenae', on either side of the head), with large short eyes. The glabella (the bulbous central portion of the cephalon between and in front of the eyes) was sloping and poorly defined. The eyes were holochroal – in Isotelus iowensis there were up to around 5000 hexagonal lenses (or facets) per eye, the largest about wide. The pair of thin genal spines at the corners of the cephalon are a diagnostic trait among species, with some species (such as I. rex and I. gigas) lacking genal spines entirely as adults, and in other species, the genal spines may reach as far back as the pygidium. The cephalon has a doublure which extrudes inwardly from the border, enclosing the underside slightly, and strongly terraced (forming step-like ridges). Attached to the doublure anteriorly was the hypostome, the trilobite mouthpart – it was large and forked in Isotelus, with 2 large posteriorly-facing prongs. The prongs were triangular in cross-section, with a basically flat ventral surface (although with a small lips around the edges), and a tall ridge on the dorsal, upwardly-facing surface. The hypostome was strongly terraced. The thorax is composed of eight segments of equal size. The segments are smooth dorsally, though marked with small pits and occasionally shallow bumps, while the short pleural lobes are marked by strongly sculpted "terraces" on their articulating facets (the surface which dips down anteriorly on the pleural lobe, and allows the segments to glide over each other when the trilobite enrolls or flexes). The axial furrow between the axial lobe and pleural lobes is shallow and smooth. The pygidium is expansive and mostly smooth, subtriangular in outline, with anterior articulating facets similar to the pleural lobes of the thorax. The axial and pleural lobes of the pygidium are poorly defined, with a shallow furrow dividing the border of the pygidium from the central portion. The border extends ventrally into a doublure, which slightly encloses the underside, and is strongly terraced.
Ventral anatomy and patterning
The limbs of Isotelus are poorly preserved, with the Ohio specimen showing at least 26 pairs of identical legs. Each of the 8 thorax segments would possess a single leg pair, with 3 beneath the cephalon, and about 16 pairs beneath the pygidium. The protopodite base of the leg was large and flattened vertically, and the subsequent podomeres of the leg were all cylindrical and slender. No evidence for endites on the legs is known. As with other trilobites, a pair of anteriorly directed antennae would have been present. The second pair of cephalic legs was possibly differeniated, being smaller and more inwardly directed towards the hypostome as an adaptation for food processing. Palaeontologist John Wells discovered probable color-markings on the dorsal shells of some specimens of Isotelus, which may hint at the original patterning. Higher concentrations of pigment are found within the axial furrows on the thorax (strongest at the anterior and posterior-most points of the axial furrows), which would have appeared in life as two dark strips running down the length of the body, extending slightly onto the cephalon and some distance onto the pygidium. The genal spines of the cephalon were darker (as they are in many trilobites with preserved pigmentation). There was a dark crescent-shaped region of pigmentation beneath the eye on the free cheek, extending slightly onto the glabella and palpebral lobe (the surface of shell which covers the eye dorsally).
Paleobiology
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. During the Ordovician-Silurian extinction event, the widespread onset of cold water conditions and anoxia may have instead favoured species that produced small numbers of large eggs, from which hatched larvae that were already benthic and adult-like in morphology. During the later meraspid stage and adulthood, Isotelus may have used emptied nautiloid shells lying on the sea floor as temporary sanctuaries for moulting, when the exoskeleton was soft and the trilobite was vulnerable. 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. The two prongs have been suggested to have acted like a claw-hammer, and in combination with burrowing, is suggested to have allowed Isotelus to pry soft-bodied worms out of sediment. Supporting this, Isotelus iowensis has, within the Maquoketa Shale, been found disproportionately in layers containing abundant Chondrites trace fossils left by worms, which they may have been feeding on. 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