Specimen description :

• Positive.
• Prone on matrix.
• Complete specimen, with a slight dissociation of the pygidium to the left.
• Total size : 18 mm.
• Middle Cambrian (-525 Ma).
• Upper part of the Glossopleura Zone, Chisholm Shale.
• Pioche, Nevada, USA.

Order

Ptychopariida

SWINNERTON 1915

• Cephalon : Generally bearing genal spines.
• Typically opisthoparian facial sutures
• Gently forward-tapering simple glabella bearing a broad, rounded front.
• Usually 3 pairs of rather narrow parallel glabellar furrows.
• Natant hypostome (some forms conterminant).
• Rostral plate shorter than the glabellar length.
• Thorax : Typically large, with 8+ thoracic segments.
• No fulcræ.
• Pygidium : Quite variable, without noticeable structures.
• Typically with a small pygidium bearing a border with terrace lines (Cambrian forms) or a larger pygidium, with or without border (post-Cambrian forms).

Suborder

Ptychopariina

RICHTER 1933

Paraphyletic primitive Ptychopariida, a large and extremely plesiomorphic group.

• Cephalon : Glabella usually tapering or barrel-like.
• Usually 3 pairs of glabellar furrows.
• Sutures typically opisthoparian (but some proparian, and blind forms marginal)
• Anterior sutures usually convergent to slightly divergent, posterior sections moderately to highly divergent.
• Eyes usually present, medial, and near glabella
• Usually blade-like genal spines present.
• Relatively narrow rostral plate.
• Thorax : Generally long, relative to pygidium.
• Pygidium : Micropygous to subisopygous.

Superfamily

Ptychoparioidea

MATTHEW 1887

Occurence : Cambrian to Ordovician. Forms with oval, elongate, exosqueleton.

• Cephalon : Semi-circular.
• Opisthoparian sutures.
• Typically with well-defined border.
• Glabella tapering forward, bearing 3 to 4 pairs of glabellar furrows.
• Preglabellar field present, convex or flat.
• Natant hypostome.
• Short to medium sized genal spines.
• Eye ridges usually present.
• Exceptions include eyeless forms (Conocoryphidae), proparian forms (Norwoodiidae), rounded genal angles and gonatoparian sutures (Menomoniidae).
• Thorax: Relatively broad.
• Typically 12 to 17 segments.
• Pygidium : Micropygous but exceptionally isopygous (e.g., some Asaphiscidae, Cooselidae).
• Transverse, pleural field nearly flat, with distinct pleural grooves;
• Few segments.
• Devoid of border.

Family

Alokistocaridae

RESSER 1939

Occurence : Lower Cambrian to Upper Cambrian.

• Cephalon : Preglabellar area wide.
• Glabella somewhat taperring.
• Facial sutures rather deeply concave opposite the glabella.
• Eye ridges well developped.
• Palpebral lobes small.
• Genal spines of moderate size.
• Pygidium : Micropygous.

Genus

Piochaspis

LEVI-SETTI 1975

• Cephalon : Anterior border convex.
• Anterior border furrow with median imbed.
• Preglabellar field depressed mesially, elevated laterally.
• Glabella tapering forward, rouded in front.
• 3 pairs of shallow lateral furrows.
• Axial furrows deep.
• Facial sutures diverging in front of the eyes.
• Eye ridges narrow, ~ 0.3 of glabellar width at midlength.
• Palpebral areas convex, narrow anteriorly.
• Posterior area of fixigena deeply furrowed.
• Librigenae with pointed genal spines, flaring posteriorly at ~ 45°.
• Thorax : 17 segments.
• Pleurae grooved.
• Pleural spines short, outward pointing.
• Pygidium : Micropygous.
• Devoid of border.
• Prominent axis.
• 1 axial ring.

Species

sellata

LEVI-SETTI 1975

Piochaspis sellata. Holotype. PE 54115. (After R. LEVI-SETTI, 1993)

Same specimen, while immersed in xylène (better contrast due to different difractions) (After R. LEVI-SETTI, 1993)