Specimen description :

• Full specimen.
• Enrolled on matrix.
• Total size : 45 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

Megistaspis (= Megalaspis)

JAANUSSON 1956

The genus include over 20 species. Occurrence : Low and Middle ( ?) Ordovician of Europe, Asia and North America ( ?). Average and large (sometimes very large) trilobites (Max length of the body of some species is up to 400 mm). The cephalon and pygidium are often triangular and very oblong. Both shield are practically of equal size.

• Cephalon : Librigenae taking part in the formation of genal angles.
• Facial sutures opisthoparian.
• Frontal shied (limb) is larger or very large.
• Glabella short.
• Facial sutures oblong in the frontal region, joining (frequently like an oblong cone) far from the glabella.
• Genal angles acute, often transforming in genal spines.
• Surface of the shield smooth, sometimes with small pits in the place of the vibrissae.
• Hypostoma uninterrupted round or acuminate behind.
• Thorax : 8 segments.
• Pygidium : Often divided in 2 by pleural ribs.

Species

lawrowi

SCHMIDT (?)

Very large to gigantic sized Megistaspis (Max. length of the body is up to 260 mm). Body fusiform and oblong. Occurrence : Lower Ordovician, Arenigian. The upper part of the Kunda Horizon and the lower part of the Azery Horizon ( ?) of the NW of Russia (neighbourood of St Petersburg).

• Cephalon : Parabolic to rounded-triangular.
• Genal angles ending with very long, thick, strongly skewed genal spines (reaching the 7th-8th segments).
• Ratio width/length : 1,5.
• Glabella oval, strongly convex, in appearance similar both to the glabella of M. hyorhina and to the glabella of M. gibba.
• Dorsal furrows distinctly expressed.
• Lateral furrows more or less shown.
• Triangular basal lobes hardly noticeable.
• Occipital furrows and occipital ring very weakly expressed.
• Anterior branches of the facial sutures short, smoothly bent, runnig almost parallel to the margin of the cephalon and forming a rather acute angle.
• Occipital node in form of of a flat broad dome.
• Eyes small, with a flat palpebral lobe.
• Distance between the eyes and the posterior margin of the cephalon is essentially greater than their diameter.
• Librigenae slightly convex.
• Thorax : Axial part narrower than the pleural one.
• Segments of the rachis in the axial part very flat.
• Pygidium : Convex, parabolic with a moderate broad limb (narrowed in the lateral parts of the pygidium).
• Rachis thin, practically without segmentation.
• Pleural parts of the pygidium practically smooth.
• Ratio width/length = 1,4.

Side view of the complete specimen

Better view of the pygidial segmentation