Order
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Asaphida
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SALTER 1864
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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.
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Superfamily
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Asaphoidea
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BURMEISTER 1843
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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).
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Family
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Asaphidæ
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BURMEISTER 1843
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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.
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Subfamily
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Asaphinæ |
BURMEISTER 1843
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- 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.
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Genus
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Megistaspis (= Megalaspis)
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JAANUSSON 1956
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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.
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Species
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triangularis
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SCHMIDT (?)
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Average and large sized trilobites (Max length of the body is up to 150
mm). Body fusiform and oblong.
Lower Ordovician, Arenigian, to the upper part of the Wolchowian horizon,
Russia (Neighbourhood of St Petersburg).
- Cephalon : Triangular, almost equilateral.
- Very long, thick genal spines, rather strongly skewed (reaching the
7th-8th segment).
- Librigenae slightly convex.
- Glabella oval, flat-convex, in the front rather flatly sloping toward
the limb.
- Dorsal furrows distinctly expressed.
- Lateral furrows not always distinct.
- Triangular basal lobes weakly advanced.
- Occipital furrow weakly expressed.
- Occipital ring flat.
- Anterior branches of facial sutures smoothly bent, running almost
parallel to the margin and forming an acute angle at the joining point.
- Occipital node not expressed.
- Eyes small.
- Strongly curved palpebral lobes.
- Distance between the eyes and the posterior margin significantly exceeding
their diameter.
- Width / length Ratio = 1,5.
- Thorax : 8 segments.
- Axial part narrower than the pleural ones.
- Axial segments flat.
- Pygidium : Convex.
- Triangular.
- Acute pygidial spine.
- Limb not broad, narrowed on the lateral parts.
- Rachis thin, vaguely segmented (nearly 20 segments).
- Sharply outlined double ribs on the pleural parts (up to 12 pairs).
- Width / length Ratio = 1,1.
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As it can been seen, the pygidium doesn't have any sign of segmentation on
the pleural parts, but its preservation was poor.