Elrathia kingi

 

This is one of the most famous trilobite of the world, as well known as the morrocan classic Flexycalymene ouzregui.

Elrathia kingi is the most ubiquitous trilobite in the Wheeler Shale of Utah, occuring at numerous sites in the House Range and Drum Moutains. The 2 specimens presented on this page are the second and third trilobites that my mother offered me as a kid, and I still do feel a particular sensation when I am looking at them. I can't count the sheer number of hours I passed staring at these 3 bugs (the other one being .... an excellent Flexycalymene ouzregui).

Ican say that this website is nothing more than the result of a gift made 25 years ago ;)

 

Specimen description :

  • Positive.
  • Free of matrix.
  • Allmost complete (95 %), only lacking a few thoracic tips and the left genal spine.
  • 13 thoracic segments.
  • Molt on the ventral side.
  • Slight disarticulation of the facial sutures.
  • Total size : 18 mm.
  • Middle Cambrien (-525 Ma).
  • Wheeler Shale.
  • Utah, USA

 

 

 

 

Dorsal view.
Ventral view of what appears to be the molt.

 

Diagnosis :

 

Order

 

 

 

 

 

Ptychopariida

 

 

 

 

SWINNERTON 1915

 

 

 

 

 A large, heterogenous order, with classification problems. Occurence : Cambrian to Upper Ordovician.
  • 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

Elrathia

WALCOTT 1924
  • Cephalon : Alokistokarid with rather wide cranidium.
  • Strong eye ridges.
  • Longitudinal striæ on a wide preglabellar area.
  • Commonly, tubercules on cranidium.
  • Pygidium : Relatively large.
  • Few segments.
  • Little developped segmentation.

Species

kingi

MEEK

A medium sized trilobite with a smooth subovate carapace that is tappered toward the posterior.

  • Cephalon : Evenly rounded anteriorly.
  • Glabella unfurrowed, slightly tapered and broadly rounded anteriorly.
  • Genal spines generaly extanding to the 2nd or 3rd thoracic segment.
  • Thorax : Usually 13 segments.
  • Pygidium : 4 axial rings.
  • Long terminal piece.
  • Posterior margin usually with a broad medium notch.

 

Complementary pictures :

 

Dorsal view, B&W.
Ventral view, B&W.

 

Synthetogram :

 

Reconstruction after F. GÜNTHER & V. GÜNTHER, 1981.

 

Complementary specimen :

Another specimen, slightly larger and better articulated, but lacking the right genal spine. Remark the broad medium notch on the posterior margin of this specimen.

Founding place : same locality.

Dorsal view. Size : 27 mm.

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