START
HERE
1
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2 or 3 thoracic segments, strongly
isopygous:
ORDER AGNOSTIDA
-- Click YES for suborders
YES
Exceptions: Orders
Corynexochida and Asaphida (Trinucleioidea) include a few species with 2
or 3 thoracic segments
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vs
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4 or more thoracic segments
-- go to 4
YES
Exceptions: Trilobites
with exactly 4 segments occur in the order Asaphida (Trinucleioidea). Immature
(meraspid) specimens of several orders may show four thoracic segments at
some time in their development.
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2
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3 thoracic segments, eyes, free
cheeks, proparian sutures:
Suborder Eodiscina
of ORDER AGNOSTIDA
YES
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vs
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2 thoracic segments, eyeless:
go to 3
YES
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3
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Pygidial axis with 3 or fewer segments,
segments not ringlike:
Suborder Agnostina
of ORDER AGNOSTIDA
YES
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vs
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Pygidial axis long, narrow, with
typically >3 ringlike segments, cephalic border thick:
Suborder Eodiscina
of ORDER AGNOSTIDA
YES
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4
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Eyes and palpebral lobes large,
crescentic; glabellar furrows simple, well-developed, subparallel; thorax
with many segments (often 14) with spinose tips; pygidium typically small
or rudimentary:
ORDER REDLICHIIDA
YES
Exceptions: Some Redlichiida (Paradoxidoidea) with subisopygous
pygidia (e.g., Xystridura); some primitive Corynexochida with up to
18 thoracic segments, and micropygous pygidia (e.g., Lancastria)
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vs
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Not with this combination of characters
-- go to 6
YES
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5
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Opisthoparian cephalic sutures:
Suborder Redlichiina
of ORDER REDLICHIIDA
YES
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vs
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Facial sutures fused (lacking),
eye lobes fused to posterolateral part of anterior glabellar lobe:
Suborder Olenellina
of ORDER REDLICHIIDA
YES
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6
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Proparian cephalic sutures; glabellar
shape expanding forward -- go to 7
YES
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vs
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Other suture types (opisthoparian,
gonatoparian, marginal, fused) -- go to 8
YES
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7
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Schizochroal eyes; sigmoid 3S (anterior
glabellar furrow); thorax of 11 segments, typically with furrows and articulating
facets:
Suborder Phacopina
of ORDER PHACOPIDA
YES
Exceptions: There are secondarily eyeless species of Phacopina
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vs
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Holochroal eyes, thorax typically
with distinctive fulcral structure, pleurae furrowed medially, simpler distally,
often spine tipped; pygidium often with lobed or spinose margin; exoskeleton
often tuberculate:
Suborder Cheirurina
of ORDER PHACOPIDA
YES
Exception: A few Cheirurina with opisthoparian sutures, pygidium
may be simple.
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8
|
Gonatoparian facial sutures, glabella
narrows forward, eyes typically small, holochroal; thoracic pleurae typically
with rounded ends, 11 - (typically) 13 segments:
Suborder Calymenina
of ORDER PHACOPIDA
YES
Exceptions: A few Calymenina have proparian or opisthoparian sutures
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vs
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Opisthoparian or marginal sutures
-- go to 9
YES
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9
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Opisthoparian cephalic sutures
-- go to 10
YES
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Marginal cephalic sutures (or sutures
not apparent) -- go to 16
YES
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10
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Exoskeleton strongly granulose
and spiny; opisthoparian cephalic sutures; glabella with complex structure
of fused lobes; thorax 10-11 segments, often spinose; pygidium large, often
longer than wide, with 3 pairs of furrowed pleurae, tips generally leaflike
and/or spinose:
Superfamily Lichoidea
of ORDER LICHIDA
YES
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vs
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Not with this combination of characters
-- go to 11
YES
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11
|
Glabella sides parallel or tapering
forwards to anterior border; lateral lobes distinct, rounded; thoracic 8-10
segments, typically with long pleural spines; pygidium short, w/ 2-3 segments
& 1or more pairs of marginal spines, largest connected to 1st pygidial
segment by prominent ridges:
Superfamily Odontopleuroidea
of ORDER LICHIDA
YES
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vs
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Not with this combination of characters
-- go to 12
YES
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12
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Glabella expanding forwards, often
to anterior border w/sides often concave or linear, glabellar furrows (if
present) splayed (anterior pair ends anterior of origin,posterior pair ends
posterior of origin), sometimes furrows reduced to pairs of pits; thorax not
long, typically 7-8 segments; pygidium relatively large, often isopygous or
subisopygous, may be spinose:
Suborder Corynexochina
of ORDER CORYNEXOCHIDA
YES
Exceptions: micropygous
in some primitive Corynexochina, thoracic segment numbers vary from 2 to 18.
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vs
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Not with this combination of characters
-- go to 13
YES
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13
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Glabella rounded in front; pygidium
isopygous, usually without marginal spines -- go to 14
YES
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vs
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Not with this combination of characters
-- go to 15
YES
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14
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Anterior portion of facial sutures
curve inward and forward to intersect cephalic margin in front of glabella,
forming ventral median suture:
ORDER ASAPHIDA
YES
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vs
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Anterior facial sutures not curved
inward toward midline (may be strongly diverging), typically 8-10 thoracic
segments:
Suborder Illaenina
of ORDER CORYNEXOCHIDA
YES
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15
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Glabella typically strongly vaulted,
with 4 or fewer pairs of subparallel furrows, anterior two pairs often very
short, basal pair long and backwardly curved; eyes typically large, holochroal;
pygidium usually large:
ORDER PROETIDA
YES
Exceptions: Proetida Superfamily Aulacopleuroidea may have non-vaulted
glabella, small eyes, numerous thoracic segments (11 to 22)
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vs
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Not with this combination of characters
-- go to 17
YES
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16
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Cephalon large, broad preglabellar
field slopes down to cephalic brim and border, eyes often reduced; thorax
with 12+ narrow segments, pygidium usually very small: Order HARPETIDA (formerly in ORDER PTYCHOPARIIDA)
YES
Exception: Some primitive Harpetida with opisthoparian sutures
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vs
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Thorax with 6 segments; pygidium wide,
and of many segments: Superfamily Trinucleioidea
of ORDER ASAPHIDA
YES
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17
|
Opisthoparian cephalic sutures;
mostly eyeless; long genal spines; glabella expanding forwards, convex; thorax
with 5-8 segments; pygidium triangular, wide, segmented:
Superfamily Trinucleioidea
of ORDER ASAPHIDA
YES
Exceptions: Some primitive Trinucleioidea with numerous thoracic
segments and no clear distinction between thorax and pygidium. Some advanced
Trinucleioidea with as few as 2-3 thoracic segments (progenetic Raphiophoridae).
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vs
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Glabella tapering forward or parallel
sided, often not reaching anterior border, typically 3 pairs simple furrows
running obliquely inward and back; eyes often small, but also long or absent;
cephalic border typically present; thorax generally of 12-13 segments (but
rarely <8 or >20); pygidium generally short, small, of 6 or fewer
segments: Suborder Ptychopariina of ORDER PTYCHOPARIIDA
YES
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