6Nov2000s

[ Back ] [ Next ] [ Home ]

6 November 2000

Dear Members of the Frasnian WG:

We appreciate the support expressed by Thomas Becker in today's e-mail for our positions on Frasnian substage boundaries. Willi Ziegler and I had been working for the past several days on a rebuttal to Igor Bardeshev's recent message to the group concerning a possible much older range of Palmatolepis semichatovae in Tadjikistan. Hence, the timing is perfect to allay any doubts that the Frasnian WG members might have, as expressed in Thom's e-mail of 27 October, concening the possible lack of synchroneity of the first appearance of Palmatolepis semichatovae. Herein, we hope to put to rest for all time the notion that Pa. semichatovae originated and lived much earlier in central Asia and then suddenly migrated worldwide with the eustatic rise in the Early rhenana Zone or its M.N. equivalent.

Recently, Igor Bardeshev wrote:

Dear colleagues,

The decision which was accepted by SDS in 1997 satisfies me only partly. I could accept tree subdivisions in Frasnian, but as conodontologist I don't like their borders by two reasons:

1. The appearance of most important for Late Devonian Palmatolepis genus is the event which is suitable for indexation even of Upper Devonian, while the appearance of Palmatolepis punctata is the event with much less scale.

2. It's not clear why the border between Middle and Upper subdivisions is accepted by Palm. semichatovae, instead of one of zonal species-index. From the beginning of 80-th many of conodontologists know that this species in Central Asia appeares much early than in other regions. In "Shishkat" section Palm. semichatovae appears in the upper part of Lower hassi zone and disappears in the lower part of jamieae zone (Bardashev I.A., Sabirov A.A., 1997; Bardashev, Bardasheva, 1999). The validity of my definitions was confirmed by author of the species - Nonna Ovnatanova.

I can't agree that our WG was formed only for selection of stratotypes for subdivisions. I think that having studied all available materials and having discussed possible variants we could find optimal decisions both on quantity of subdivisions and on their borders.

These misstatements cannot go unchallenged, so herein Willi Ziegler and I provide the following joint rebuttal and correction:

1. Here, Igor seems to be arguing for the use of the first Palmatolepis species, Palmatolepis transitans, as a Substage boundary. Unfortunately, the first appearance of this species does not coincide with any significant eustatic or other event or with any great change in marine biotas. Moreover, Pa. transitans is a species open to diverse interpretations by a number of conodont workers. Inasmuch as Mesotaxis falsiovalis, its direct ancestor, develops a Palmatolepis-like carina long before widening of the platform, or posteriorward migration of the basal cavity and its transformation to a pit beneath the central node, these differences of opinion as to when Mesotaxis becomes Palmatolepis are understandable. Palmatolepis punctata, on the other hand, is widely recognized as the first species of Palmatolepis to develop a broad lobe, clearly separated from the platform outline by anterior and posterior notches. Moreover, the first appearance of this species coincides with a major transgression that produced significant changes in the marine biota.

2. The statement that Palmatolepis semichatovae appears in central Asia in the upper part of the Lower hassi Zone and disappears in the lower part of the jamieae Zone can now be disregarded as totally erroneous. The fact is that Pa. semichatovae in Igor's Shishkat section appears and disappears within the Early rhenana Zone, matching its range everywhere else in the world. We now know this to be a fact after jointly and carefully reexamining the plates that Igor had shown us in 1998 and kindly left with Willi. With Igor, we had looked only at the Pa. semichatovae and agreed with his determination. Now, we examined the other taxa that he illustrated as occurring with it. They represent an unequivocal Early rhenana Zone fauna and include: Pa. barba (another species restricted to the Early rhenana Zone), Pa. rhenana nasuta (the definitive taxon for this zone), and the late morphotype of Pa. proversa that first appears in the Early rhenana Zone. Hence, the first appearance of Pa. semichatovae, which coincides with perhaps the greatest Frasnian transgression, is still synchronous globally and is at the same position in both the standard and M.N. zonations.

Charles A. Sandberg and Willi Ziegler