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The emergence and evolution of linograptids Adam URBANEK In: Urbanek, A. & Teller, L. (eds.), Silurian Graptolite Faunas in the East European Platform: Stratigraphy and Evolution. Palaeontologia Polonica 1997, 56

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Abstract:

Macroevolutionary effects due to anagenetic changes within a single lineage have been traced in the
Linograptinae from Late Wenlock single stiped Lobograptus? sherrardae, through bipolar
Neodiversograptus and multibrachiate Linograptus, to Early Devonian compound colonies of Abiesgraptus.
The ability to generate a number of sicular cladia improved the stability of the rhabdosome, and, in the
evolution of Linograptus, the resulting geometrical constraints were overcome by diverting their growth.
The main lineage is subdivided into four chronospecies displaying periods of fairly high abundance and
a distinct morphological norm. These periods were separated by intervals of comparative scarcity,
probably related to the bottlenecking of the lineage. The only instance of change in large populations is
the L. posthumus A. tenuiramosus phyletic transition. Speciation events (transformations) seem to be
governed by the mechanisms of transient polymorphism, whilst periods of relative persistence of the
norm were probably related to microevolutionary changes in monomorphic populations. A disparate
origin of unit characters within the complex Abiesgraptus pattern is suggested. These characters
appeared independently and heterochronously within the populations over the entire range of
distribution of the ancestral species to be later assembled into a single adaptive syndrome.
Cosmopolitan distribution of both the ancestral and the daughter species implies a polycentric or a
pantopic emergence of the Abiesgraptus.

Key words: Linograptinae, multiramous colonies, phyletic evolution, macroevolution, adaptive syndrome.

Adam Urbanek, Instytut Paleobiologii PAN, ul. Twarda 51/55, 00 818 Warszawa, Poland. urbanek@twarda.pan.pl