Asaphus lepidurus

 

 

This is a trilobite occuring in the Lower Ordovician, Arenigian, of the North-West of Russia. This is one of the 35+ species of the Asaphus genus which nicest specimens are collected and prepared by and at the Paleolab of St Petersburg. Historically, most of the Asaphids have been found and described in Northern Europe, but the exceptional preservation of the russian specimens lead many collectors to renew their former specimens.

Asaphus lepidurus can be collected in North-West of Russia in the middle part of the Wolchovian Horizon (Neighbourhood of St Petersburg and Kaliningrad), in Estonia and in the North of Poland (Neighbourhood of Gdansk).

This is the second specimen I do present.

 

Specimen description :

  • Full specimen.
  • Prone on matrix.
  • Total size : 75 mm
  • Lower Ordovician, Arenigian.
  • Wolchovian Horizon
  • Wolchow River, St Petersburg.
  • Russia.
Dorsal view of the positive part.

 

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

Asaphus

BRONGNIART 1822

Average sized trilobites. Occurence : lower and middle Ordovician, Europe and Asia.

  • Cephalon : Semi-circular or rounded-triangular.
  • Opisthoparian.
  • Librigenæ partially form genal angles.
  • Genal angles commonly rounded.
  • Posterior border of the genæ convex.
  • Glabella most often pyriform, more or less convex, reaching the anterior branches of the facial sutures.
  • Facial sutures nearly blend with the anterior margin of the cranidium.
  • Hypostoma has a dichotomy in the direction of the posterior parts.
  • Pygidium : Isopygous.
  • Semi-circular or rounded-triangular.
  • Indistinct smooth lateral ribs.
  • Doublurea in the anterior part does not reach rachis.

Species

lepidurus

NIESZKOWSKI (?)

 

Average to large sized Asaphus (max : 120 mm). Occurence : Lower Ordovician, Arenigian. Body more or less oval.

  • Cephalon : Rounded-triangular.
  • Relatively acute genal angles.
  • Width / length = 2.05
  • Occipital furrows clearly expressed, gradually transforming into the posterior marginal furrows.
  • Glabella pyriform.
  • Expressed lateral glabellar furrows and fossulæ (interocular pits).
  • Poorly developped occipital node.
  • Eyes small, distance between the eyes and the posterior margin of the cephalon exceeding their diameter.
  • No terrace lines on the librigenæ.
  • Thorax : Axial part slightly narrower than the pleural one.
  • Axial part covered with thick terrace lines,
  • Pygidium : Rachis disctinctly segmented
  • Thin lines running from the rachis to the border rim on the pleural part, often dichotomic..
  • Width / length = 1.65

 

 

Complementary pictures :

 

Dorsal view of the cephalon
Dorsal view of the pygidium.

 

 

 

Frontal view of the cephalon.

 

Side view of the complete specimen

 

Terrace lines on the pleural parts of the pygidium.

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