Specimen description :

• Complete specimen lacking the right eye and a bit of the pygidial border.
• flat on matrix.
• Total size : 38 mm
• Upper Ordovician. Arnheim Formation.
• Highland County, Ohio

Order

Asaphida

SALTER 1864

Occurrence : Middle-Upper Cambrian boundary to upper Ordovician-lower Silurian.

• Cephalon : Opisthoparian.
• Often equal / subequal to pygidium (e.g., Asaphoidea), but some not so (e.g., Trinucleioidea).
• Usually with a high degree of cephalic effacement so glabellar furrows are faint or not visible.
• Eyes usually large (some forms secondarily blind).
• Preoccipital glabellar tubercle in late forms.
• Cephalic doublure often wide, with terrace ridges.
• Librigenæ are typically separated by a median ventral suture.
• Dorsal anterior facial sutures often curve adaxially to meet in front of the glabella.
• Hypostome conterminent or impendent, with only primitive forms (e.g., the Anomocaroidea) natant.
• Thorax : Typically 5 – 12 segments, but 2 - 3 in a few Trinucleioidea, 13+ in some Anomocaroidea, up to 30 in an Alsataspidid (Trinucleioidea).
• Pygidium: Typically large (subisopygous to macropygous).
• Wide doublure.

Superfamily

Asaphoidea

BURMEISTER 1843

Occurrence: Middle Cambrian to Ordovician.

• Cephalon : Preoccipital glabellar tubercle.
• Glabella elongate, subparallel to tapering forward.
• Defined occipital ring.
• Curved, apostrophe-like pair of basal glabellar furrows isolated within glabella.
• Hypostome conterminant, fixed to the doublure (rarely impendent)
• Thorax : 6 - 9 segments, typically 8.
• Pygidium : Typically rounded.
• Typically without spines, sometimes with a terminal spine or pair of spines (e.g., Thysanopyginae).

Family

Asaphidæ

BURMEISTER 1843

Family tends toward loss of apparent segmentation of cephalon and pygidium, obsolescence of axial furrows and deep notching of posterior margin of hypostoma.

• Cephalon : Librigenæ separated anteriorly by a median suture.
• Asaphoïd with well defined to obsolete glabella, considerably longer than frontal area.
• Lateral glabellar furrows mostly weaks or absent.
• Most genera with distinct glabellar tubercule.
• Eyes generally somewhat distant from axial furrows.
• Faint, almost obsolete eye ridges only know in 2 genera.
• Doublure commonly broad.
• Genal spines generally short and with a wide basis.
• Posterior margin of hypostoma varying from pointed (later forms) to deeply notched and/or with panderian openings.
• Thorax : 8 segments.
• Pleural furrows generally diagonal, if present.
• Panderian organs developped as notches or separate openings, but absent in some (e.g., Ogygiocaridinæ, Symphysurininæ).
• Pygidium : External margin varying from rounded to pointed.
• Some genera with terminal spine.

Subfamily

ANGELIN 1843

• Cephalon : Glabella, if defined, slightly tapering forward, parallel-sided or somewhat expanding in front of the eyes.
• Glabellar tubercles immediately in front of or at some distance from occipital furrow or of area corresponding to it. Distance between this tubercle and occipital furrow (with some exception) smaller than length (sag.) of occipital ring.
• Anterior wings of hypostoma more or less triangular.
• Panderian organs generally developed as separate openings.
• Ornementation of terrace lines mostly poorly developed or absent.

Members of the group A :

• Posterior margin of hypostoma strongly concave to deeply notched (including genera in which hypostoma is still unknown, but which probably have similar shape of hypostomal posterior margin).

Genus

Isotelus

DEKAY 1824

Species

maximus

LOCKE 1838

• Cephalon and Pygidium : Semi-circular in outline.