Megistaspis sp.

 

 

Here is a Megistaspis for which I am waiting a precise identification from the Paleolab. I bought this bug from an US seller that sold it to me as Megistaspis heroica :) Well, the size and the quality of the preparation deserve such a nickname to me... ;)

Megistaspis (= Megalaspis) includes over 20 species in Europe, Asia and North America. They are average to large (sometimes gigantic) trilobites (Max length of the body of some species is up to 400 mm). This specimen has been collected et prepared by and at the Paleolab of St Petersburg .

 

Specimen description :

  • Full specimen.
  • Prone on matrix.
  • Total size : 125 mm.
  • Middle Ordovician ?
  • Asery level ?
  • Wolchow River, St Petersburg ?
  • Russia.
Dorsal view of the specimen.
Lateral view of the specimen.
Frontal view of the cephalon.

 

Diagnosis :

 

Order

 

Asaphida

 

 

SALTER 1864

 

 

 Order including 1/5 of all the species of Trilobites, it mostly regroup librostoms of various morphology, in which the most advanced families do present a ventral median suture early visible in the ontogeny ( "asaphoïd" protaspid larvæ).

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

 Asaphinæ

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

sp.

(?)

 

 

Discussion :

Megalaspis ANGELIN 1851 is now considered as a synonym of Megistaspis JAANUSSON 1956 .

Complementary pictures :

 
Dorsal and frontal view of the cephalon.
Dorsal and lateral view of the pygidium.

 

side view of the cephalon.

 

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