SCIENTIFIC PORTAL FOR ALL USERS INTERESTED IN PTEROBRANCHS AND GRAPTOLITES
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New observations on the fine structure of graptoblasts
Adam URBANEK, Piotr MIERZEJEWSKI & Barrie RICKARDS
Lethaia 1986, 19, 97-107
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ABSTRACT:
Graptoblasts are redefined as widenings of the sealed terminal portions of graptolite stolothecae resulting in ovoid
bodies composed of an outer fusellar layer, named here as blastotheca, and a inner secondary layer of electron dense
material, named the blastocrypt. In their proximal portion a number of graptoblasts display a scar with fuselli stripped
and the crassal material of the blastotheca exposed and displaying a rough and dull surface. The boundary of this area,
termed the talus, is edged by a distinct escarpment, probably the margin of the preserved part of the blastotheca. The
outer wall of the talus is usually abruptly terminated. Only exceptionally is the outer wall conical and gradually widening.
Thus the transverse partition between the parental stolotheca and the graptoblast was produced by the blastocrypt
without fusellar covering. It seems, therefore, that graptoblasts were more probably lacking any internal portion of their
blastothecae which could possibly be compared with the internal parts of a dendroid autotheca. It is concluded that
graptoblasts housed a zooid that never functioned as an autozooid, but suffered arrested development and encystation.
The biological role of graptoblasts as dormant bodies is discussed.
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