Specimen description :

• Full specimen.
• Prone on matrix.
• Total size : 95 mm
• Lower Ordovician, Llanvirn.
• Asery level.
• Wolchow River, St Petersburg.
• Russia.

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

BURMEISTER 1843

• Cephalon : Glabella commonly expanded in front of eyes.
• Posterior lateral furrows commonly strong, obliquely directed, mostly deeper than part of axial furrows laterally delimiting posterior lateral glabellar lobe.
• Glabellar tubercle situated immediately in front of occipital furrows or of area corresponding to this furrow.
• Posterior border furrow generally distinct.
• Panderian organs developped as notches or separate openings.
• Anterior wings of hypostoma broad (tr.), more or less quadrangular in outline.
• Posterior margin of hypostoma with deep notch (except Aulacoparia).
• Pygidium : Ribs of pleural field unfurrowed, if present, or rarely with faint furrows.
• Posterior margin rounded.
• Without spine.

Genus

Platyasaphus

IVANTSOV 2000

Very large to large trilobites (Max. body lenght is up to 200 mm). Exoskeleton broad. Panderian organs on the thorax resemble closed openings, those on the librigenae are slit-like or closed.

• Cephalon : Cranidium medium-sized and shortened.
• Basal lobes present.
• Eyes low.
• Librigenal spines usually present but sometimes blunt.
• Muscle insertions areas on cranidium absent or represented by ill-defined excavations.
• shortened.
• Pygidium : shortened

Species

platyurus

(ANGELIN 1858)

Occurrence : Middle Ordovician, Llanvirn.

Very large to large Trilobites (Max. body lenght is up to 200 mm).

• Cephalon : Crescent-like.
• Genal lines turning into more or less long genal spines (for some subspecies (?) or specimen) reaching the 6th thoracic segment.
• Width /length ratio = 2,2.
• Glabella pyriform, flattened with indistinct structural elements.
• Occipital furrows poorly developped.
• Occipital ring strongly flattened.
• Eyes very similar to A. laticaudatus in form and size.
• No terrace lines.
• Thorax : Axial part narrower than the the pleural one.
• Segments broad, strongly flattened, with very poorly developped pleural furrows.
• Pygidium : Broad, somewhat truncated in the posterior part.
• Segmentation of the rachis absent.
• Hardly noticeable furrows on the pleural part.
• Width/length ratio = 1,9.

Side view of the complete specimen