| On the nature and development of graptoblasts |
| Acta Palaeontologica Polonica 2000, 45, 3, 227-238 |
Two specimens of graptoblasts, which provisionally may be assigned to
'Graptoblastoides' sp. and 'Graptoblastus' sp., are described from the early
Llandovery of Anticosti Island (Quebec, Canada). Previous reports of Silurian
graptoblasts were restricted to a single specimen from Poland. One of the new
specimens is morphologically distinct from all known graptoblasts - both chambers of
the inner cavity contain numerous incomplete septae, here referred to as hemiseptae.
SEM observations show distinct two-layered character of the transverse septum,
whereas the hemiseptae show no trace of layering. Hemiseptae and the transverse
septum are derivatives of the inner lining of the graptoblast.
The 'genera' Graptoblastus Kozlowski, 1949 and Graptoblastoides Kozlowski, 1949 are interpreted as
stages in graptoblast development, and are consequently replaced by the purely descriptive terms
'graptoblastus' and 'graptoblastoides".
The following stages of graptoblast development are recognized:
(1) pre-graptoblastoides stage,
(2) early graptoblastoides stage,
(3) late graptoblastoides stage,
(4) early graptoblastus stage, and
(5) late graptoblastus stage.
A graptoblastus s.s. (i.e. , devoid of a blastotheca) and its developmental stages
strikingly resemble the earliest stages of the astogeny of the modern pterobranch
Rhabdopleura. These observations support A.Urbanek's hypothesis that zooids
became encysted within graptoblasts and rejuvenated by a process of metamorphosis.
'Graptoblastoides' sp. and 'Graptoblastus' sp., are described from the early
Llandovery of Anticosti Island (Quebec, Canada). Previous reports of Silurian
graptoblasts were restricted to a single specimen from Poland. One of the new
specimens is morphologically distinct from all known graptoblasts - both chambers of
the inner cavity contain numerous incomplete septae, here referred to as hemiseptae.
SEM observations show distinct two-layered character of the transverse septum,
whereas the hemiseptae show no trace of layering. Hemiseptae and the transverse
septum are derivatives of the inner lining of the graptoblast.
The 'genera' Graptoblastus Kozlowski, 1949 and Graptoblastoides Kozlowski, 1949 are interpreted as
stages in graptoblast development, and are consequently replaced by the purely descriptive terms
'graptoblastus' and 'graptoblastoides".
The following stages of graptoblast development are recognized:
(1) pre-graptoblastoides stage,
(2) early graptoblastoides stage,
(3) late graptoblastoides stage,
(4) early graptoblastus stage, and
(5) late graptoblastus stage.
A graptoblastus s.s. (i.e. , devoid of a blastotheca) and its developmental stages
strikingly resemble the earliest stages of the astogeny of the modern pterobranch
Rhabdopleura. These observations support A.Urbanek's hypothesis that zooids
became encysted within graptoblasts and rejuvenated by a process of metamorphosis.
Diagrams illustrating the
developmental stages of the
graptoblast.
A. Pre-graptoblastoides stage.
B. Early graptoblastoides stage.
C. Late graptoblastoides stage.
D. Early graptoblastus stage.
E. Late graptoblastus stage.
Related pages:
Possible cellular tissue in an
Ordovician graptoblast
Crustoid graptolites (Crustoidea)
developmental stages of the
graptoblast.
A. Pre-graptoblastoides stage.
B. Early graptoblastoides stage.
C. Late graptoblastoides stage.
D. Early graptoblastus stage.
E. Late graptoblastus stage.
Related pages:
Possible cellular tissue in an
Ordovician graptoblast
Crustoid graptolites (Crustoidea)
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