Hypagnostus parvifrons Trilobite
House Range, Marjum formation of central Utah
Cambrian Period (570-540 million years ago)
Trilobites of the order Agnostida are mmall trilobites (usually only a few mm long) with cephalon and pygidium strongly similar in outline and size (isopygous); enrollment typical. Cephalon: cephalic shield with deeply parabolic outline, maximum width usually anterior of genal angle, sutures proparian or lacking; border convex; glabella fusiform, widest at base (except in Condylopygidae), glabellar segmentation highly variable, sometimes complex, but in some species entirely effaced; most species eyeless; hypostome natant, specialized with ribbon-like wings; rostral plate lacking (or uncalcified). Thorax: segments 2 (Agnostina) or 3 (some Eodiscina), axis typically broad, short fulcrate pleurae. Pygidium: strongly isopygous; pygidial margin typically closely matching cephalic margin. Occurrence: L. Cambrian to U. Ordovician (Ashgill).