Abstract

Ordinary General Meeting

7 June I972 at 4.30 p.m.

The mobility of graptolites: aspects of vertebrate palaeontology

I. More thoughts on the automobillty of the graptolltes: N. H. Kirk, University College of Wales.

This paper was published in the Journal for March x972 (J. Geol. Soc. Lond., xa8, part 2, pp. x27-I33).

DISCUSSION

Dr I. Strachan said two problems arose from the diagrams shown at the meeting. Firstly, the figure Skiagraptus (fig. Ih in the paper) with its thecae opening proximally looks exactly like the distal part of the rhabdosome of Petalograptus which could have a further distal ‘float’ or, as Dr. Kirk would have it, a ‘lower’ balancing device. We require very similarly shaped thecae to house zooids capable of producing exactly opposite current effects. This may be so but casts some doubt on the validity of the arguments used other places. Secondly, observation of diplograptid growth stages strongly suggests that spines the sicular aperture (apart from the virgella) develop when the first bud appears and can best be interpreted as gravitational balancing structures keep the sicula vertical while the first theca tends make the proximal end asymmetric. Young growth stages also show that the virgella is usually much heavier structure than the nema so that unattached and free-floating the sicula would more stable with the virgella downwards than upwards.

The Author replied, Dr. Strachan has drawn attention to an important weakness in my theory, but I am aware of this. In I969 I stated that on the theory

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