Fossil snails of the genus Trochactaeon from Kainach near Voitsberg, Styria, Austria (Gosau-Group of Kainach, upper Cretaceous) - Summary of this years prospection

[FranzBernhard]

Fossil snails of the genus Trochactaeon from Kainach near Voitsberg, Styria, Austria (Gosau-Group of Kainach, upper Cretaceous) - Summary of this years prospection

 

Introduction

Snails of the extinct genus Trochactaeon (formerly part of the genus Actaeonella) are among the most familiar fossils of the upper Cretaceous Gosau-Group of the Austrian Alps. The rather large size of some species (>10 cm), their intriguing spiral pattern in transverse sections and plenty supply, based on many mass occurrence, make them particularly popular. Some well known occurrences in Austria, distributed over several 100 km, are Brandenberg in Tyrol, “Schneckenwand”/Rußbach in Salzburg, Waaggraben near Hieflau in Styria and “Schneckengartl”/Dreistetten in Lower Austria; all of these are located within the Northern Calcareous Alps, mainly composed of Mesozoic rocks, especially Triassic platform carbonates.

The most extensive occurrence of Gosau-Group sediments in Austria, the Kainach Gosau, however, is resting on sediments of the Palaeozoic of Graz. The Gosau-Group of Kainach consists mainly of coarse- to fine-grained clastic sediments (conglomerates to siltstones, Geistthal-formation, Afling-formation), some bituminous marls (St. Pankrazen-formation) and some hydraulic marls (St. Bartholomä-formation). The age of the whole group is considered to be mostly Campanian, but stretches into the Santonian and possibly into the Maastrichtian (Ebner & Rantitsch, 2000).

In contrast to many other Gosau-Group occurrences, the Kainach Gosau is considered to be rather poor in fossils. Noticeable fossil occurrences are rudists of the St. Bartholomä-formation, some plant fossils and accumulations of small gastropods within the St. Pankrazen-formation and a few scattered ammonite concentrations within the Afling-formation. All of these fossils are already known since the 1850ies. This is also the case for the Trochactaeon snails.

 

Transverse section of Trochactaeon giganteus from the Kainach Gosau.

 

Generalized geological map of Styria with Trochactaeon occurrences in the northern part of the Kainach Gosau. Location of the Waaggraben site is also indicated.

 

Brief history of Trochactaeon in the Kainach Gosau

Trochactaeon snails were first mentioned in a footnote by Morlot (1850), thereby proofing the Cretaceous age of these sediments. Only two years later, their  existence was already doubted (Peters, 1852). However, in 1871, Stur was able to prove the occurrence of Trochactaeon in the Kainach Gosau with museum specimens already submitted by Morlot (Locality “Am Sengsenwerk `in der Eben´, Kainach, Nord”). Indicative was the host rock of the snails, which is different to the host rocks of Trochactaeon snails within the Gosau sediments of the Northern Calcareous Alps.

It took about 100 years, before several occurrences of this snail within actual outcrops were discovered by systematic investigations of a local teacher and collector in the 1960ies. But the in-situ occurrences where never described, only briefly mentioned in mapping reports and summaries of the regional geology (Gräf, 1975). The only exception is a large outcrop at the main road in Gallmannsegg north of Kainach, where some of these snails are very firmly embedded in a very hard, conglomeratic sandstone and can therefore be observed “permanently”. This site is featured in a recent excursion guide (Hubmann & Gross, 2015) and very interestingly, this is also the discovery locality of Morlot (1850), though highly modified during later road construction.

 

Discovery site of Morlot (1850), highly modified during road construction (red X). Römaskogel Mt. (1006 m) to the upper right.

 

Field work and results

Already since several years on my wish list, it took two events in March 2020 which allowed me to prospect efficiently for this snails: First was a hint from Hans Eck (Voitsberg), who pointed out some occurrences to me, some very detailed hints, some quite general. Their distribution enabled me to restrict the prospecting area to a rather small stretch of land in the northwestern part of the Kainach Gosau, namely from Gschmurgraben/Anesbach to the east to Eckwirt to the west. The second incident was the C-thing, which allowed me to make for several day trips in this area, walking along forest roads and other paths some whole days long...

 

Excerpts of the geological maps 1:50.000 Köflach (left) and Voits-berg (right) with the investigated area. All Trochactaeon occurrences are located within the red rectangle. Anesbach to the upper right, Eckwirt to the lower left, size of squares is 2x2 km.

 

This prospection resulted in more than 10 “new” occurrences of Trochactaeon snails. They range from a few snails stuck within the driveways of forest roads to up to a 0.5 m thick bed tightly packed with snails. They are situated in the uppermost Geistthal-formation or the lowermost Afling formation; the literature gives ambiguous attribution of the snail-bearing zone.

The especially good exposures along a forest road in the area of Breitenbach allowed the recognition of at least 6 Trochactaeon-bearing beds within a sediment thickness of about 20 m. About 100-200 higher in the sedimentary column, another Trochactaeon bed occurs. The host rock of the snails is a rather hard, dark grey to dark brown, mostly slightly conglomeratic sandstone, firmly enclosing the snails. Adjoining rocks of the snail beds are grey to greenish grey siltstones and sandstones, sometimes containing plant debris. Coarse-grained conglomerates are also abundant. Bedding planes dip generally with 20°-60° toward southeast to south.

Rather surprising was the discovery of nearly black, up to 1 m thick limestone lenses with abundant fragments of radiolitid rudists near the snail beds in several spots.

Continued...

[FranzBernhard]

Outcrop at a forest road with a Trochactaeon-bearing bed (between the red lines). C = main collecting area, L = Lens of black, fossiliferrous limestone. Breitenbach, point 11, 09/20/2020.

 

Detail of Trochactaeon-bearing bed at Breitenbach, Point 11 with the main collecting spot as indicated in the pic above. The roundish objects, especially near the pen, are Trochactaeon snails, some transverse sections can also be seen. 09/20/2020.

 

Outcrop of a ca. 0.5 thick Trochactaeon-bearing bed at a forest road (between the red lines). C = main collecting area. Breitenbach, point 17, bed 4, 09/20/2020.

 

Detail of Trochactaeon-bearing bed at Breitenbach, point 17, bed 4 (see pic above). The roundish objects are Trochactaeon snails, a fractured snail can also be seen (upper right). Number denotes a specimen figured later in this topic. 09/20/2020.

 

Host rock and state of preservation of the snails

 The snail-containing sandstone consists mostly of quartz grains, cemented by calcite. Larges, bright clasts are mostly dolomitic, dark clasts are composed of schists, quartz-rich rocks and lydites, as well as some greenstones. Most abundant fossils are complete Trochactaeon snails or fragments; in some specimens, abundant small, dark shell fragments of oysters (?) also occur abundantly. Rare fossils are fragments of phaceloid coral colonies and small other gastropods.

The calcitic components – fossils as well as rock clasts – are tightly interlocked with the calcitic matrix due to pressure solution. Microscopically, quartz grains seem to be embedded in shells and there is no clear-cut dividing line between fossils and matrix.

This feature results in the fact, that snails cannot be liberated properly from fresh rocks. Snails in such rocks break randomly or spall at the weak zone between two whorls. During weathering, however, a crack forms closely to the fossils, enabling their removal from the rocks; these snails appear to be covered with a sand layer. Extensive weathering may results in partial dis-solution of snails and also their disintegration. But sometimes, quite well naturally “prepped” and nearly complete snails also occur.

 

Transverse section of a Trochactaeon snail from Breitenbach, point 17, Kainach near Voitsberg. 1: End of next-to-last whorl. 2: Broken and weathered/abraded end of last whorl, it originally ended somewhere near 1, and was broken and abraded already before embedding into the sediment. 3: Contact zone between last and next-to-last whorl, where shells preferably spall during recovery. 4: Spalling along the contact zone between whorls during embedding. Collected 05/17/2020.

 

without Nr.: Unweathered rock specimen with cleanly fractured Trochactaeon snails.

4727: Strongly weathered and slightly dissolved, nearly transverse natural break through a snail.

4715: Snail with completely sandy surface, to the left and to the right some host rock.

4729: Snail with some spalling of the shell along the last and next-to-last whorl. See also pic above (AN4313).

4728: Snail with shell spalled all around.

4696: Nearly completely weathered and naturally “prepped” snail with few remnants of host rock.

           This snail is also somewhat compressed.

Continued...

[FranzBernhard]

Taxonomy

No systematic paleontological description exists of the Trochactaeon snail of the Kainach Gosau. In the 19^th century, they were all considered to be Trochactaeon giganteus (Sowerby, 1832). After the abundant finds in the 1960ies, Gräf (1975) gave the following list of species:

 

The age determination of the snail-bearing zone is based on these Trochactaeon species and the work of Kollmann (1967) and considered to be upper Santonian to lower Campanian.

My own taxonomic approach is solely based on the measurements of total height, maximum diameter and height of the spira. Measurements given in Kollmann (1967) for different species are used for reference. Most of the snails are within the proportional limits of Trochactaeon giganteus; only a few specimen are outside the proportions of this species and may belong to one of the species mentioned by Gräf (1975).

 

Two nearly complete and naturally “prepped” Trochactaeon snails, mostly still covered with a thin layer of sandstone matrix, but some shell is already visible. Specimen 4781 can be seen in-situ higher up in this topic.

 

Naturally weathered transverse section of a Trochactaeon snail and a naturally “prepped”, loose snail with only few remnants of sandstone matrix.

 

Accompanying fauna

 Small oysters (?) are the most abundant other fossils, some specimens are full of fragments of their dark to nearly black, calcitic shell fragments. These shells are often wavy and appear platy-lamellar. In several cases it appears that they have grown onto Trochactaeon snails. Cross-sections are somewhat roundish and if somewhat complete, they appear to be shallow beakers.

Fragments of phacelloid coral colonies are rare and usually very poorly preserved; some of them occur within Trochactaeon snails together with rock clasts.

Two tiny, ghost-like gastropods of another genus as Trochactaeon were also observed in polished slabs.

 

Abundant oyster (?) fragments in the matrix near a transverse section of a Trochactaeon snail.

 

Vertical section of a big Trochactaeon snail with fragments of oysters (?) and a fragment of a poorly preserved phaceloid coral colony (within last whorl of both sections). Note also the black, angular lydite clast pressed into two whorls of the shell in the left section; this is caused by pressure solution of the shell due to some compaction.

 

Oysters (?) as partly shallow, beaker-like structures growing on Trochactaeon snails (within white polygones). The nearly circular objects in the middle left pic are probably the same; a Trochactaeon shell is only very slightly below the polished surface. Breitenbach, collected from April to May 2020.

Continued...

[FranzBernhard]

Both sides of a ca. 13 mm thick polished slab from the uppermost part of bed 4 at point 17 in Breitenbach. The specimen is oriented, “top” is the upper end of this about 0.5 m thick snail bed. Besides large parts of Trochactaeon with geological levels (spary calcite within the snails), the specimen contains abundant dark fragments of oysters (?). Rock clasts are bright, well-rounded dolomite pebbles and some dark, quartz-rich clasts.

 

Fragment of phacelloid coral colony near Trochactaeon snails. This was submitted for FOTM May 2020.

Continued...

[FranzBernhard]

Vertical section of a Trochactaeon snail with a single corallite within the whorls. The corallite is visible also in the opposite section.

 

Vertical section of an unidentified gastropod with Trochactaeon snails.

 

Life style

Trochactaeon snails lived in shallow subtidal marine settings on fine-grained, soft substrates from the late Cenomanian to the early Campanian (ca. 100-80 million years before present). They tolerated some reduction in salinity and where probably deposit feeders (Kollmann, 2014). Typically, they occur in large accumulations and it is not always clear, if such accumulations represent original crowded habitats or secondary concentrations resulting from winnowing, storm events etc. The somewhat conglomeratic host rocks of the Kainach occurrences indicate, that the snails are most probably allochtonous, that means, they no longer occur in their original environment.

 

Acknowledgements

 Hans Eck is gratefully acknowledged for initiating this work with his generous hints. Fritz Messner and Heinz Kollmann are thanked for various comments.

 

References

 Ebner, F. & Rantitsch, G. (2000): Das Gosaubecken von Kainach – ein Überblick. Mitteilungen der Gesellschaft der Geologie- und Bergbaustudenten Österreichs, 44, 157-172.

Gräf, W. (1975): Ablagerungen der Gosau von Kainach. In: Flügel, H.W. (1975): Die Geologie des Grazer Berglandes. Mitteilungen der Abteilung für Geologie, Paläontologie und Bergbau am Landes-museum Joanneum, SH1, 83-1002.

Hubmann, B. & Gross, M. (2015): The vicinity of Graz: Upper Silurian to upper Carboniferous of the Graz Palaeozoic, upper Cretaceous of the Kainach Gosau and middle Miocene of Gratkorn. Berichte der Geologischen Bundesanstalt, 111, 41-74.

Kollmann, H. (1967): Die Gattung Trochactaeon in der ostalpinen Oberkreide. Zur Phyllogenie der Actaeonellidae. Annalen des Naturhistorischen Mu-seums Wien, 71, 199-261.

Kollmann, H. (2014): The extinct Nerineoidea and Acteonelloidea (Heterobranchia, Gastropoda): a palaeobiological approach. Geodiversitas, 36, 349-383.

Morlot, A. (1850): Einiges über die geologischen Verhältnisse in der nördlichen Steiermark. Jahrbuch der k.k. geologischen Reichsanstalt, 1, 99-124.

Peters, C. (1852): Beitrag zur Kenntnis der Lagerungsverhältnisse der oberen Kreideschichten an einigen Localitäten der östlichen Alpen. Abhand-lungen der k.k. geologischen Reichsanstalt, 1, Nr. 2., 1-20.

Stur, D. (1871): Geologie der Steiermark. Erläuterungen zur geologischen Übersichtskarte des Herzogthumes Steiermark. Geognostisch-montanis-tischer Verein für Steiermark, 654 Seiten.

 

Still continued...

[FranzBernhard]

And last but not least - some plates to show most of the polished ones together:

 

Transverse sections of Trochactaeon snails from Kainach near Voitsberg, collected from April to June 2020.

 

Vertical sections of Trochactaeon snails from Kainach near Voitsberg, collected from April to June 2020.

 

Vertical sections of Trochactaeon snails from Kainach near Voitsberg, collected from April to June 2020. Note the compression of AN4319 and AN4320.

 

Now, that´s really all for now . Thanks for your attention and hope, you enjoyed .

Franz Bernhard

[will stevenson]

Wow, what a treasure trove of information, maybe this should be pinned by admins? Thanks for taking us on a journey with you @FranzBernhard I have been waiting to say this as I didn’t want to interrupt the flow of your posts

[Nimravis]

Great trip report Franz. I love the detail and the way you do your pictures, and your finds are outstanding. Thanks for taking the time to put it together.

[FossilNerd]

These are the kind of educational posts that really benefit the other forum members! Even if one never gets a chance to hunt for these giant gastropods, you still have shown a great example of what goes into researching, scouting, and eventually finding fossils. I thoroughly enjoyed it. Thanks so much Franz! 

[Ludwigia]

Thanks very much for this educational report, Franz!

[marguy]

Thanks for sharing all this information and photos! Congratulations on the work done with seriousness, passion and enthusiasm. it's a real pleasure to learn with such explanations, thank you again, Franz!

[Tidgy's Dad]

So educational and informative. Fascinating stuff.

And beautiful specimens, nicely photographed too. 

[jpc]

wow... great report.  Inspirational.  

[FranzBernhard]

Thank you very much for all your kind words, @will stevenson, @Nimravis, @FossilNerd, @Ludwigia, @marguy, @Tidgy's Dad, @jpc

 

I am doing this kind of reports for my own, but its ok when you like them .

 

More seriously, I have already made an extensive report in German about this subject (about 50 pages):

https://www.franzbernhard.lima-city.de/Kainach_Trochacateon.html

 

But I would also like to have a short summary. And I was asked for an article for a local journal. So I made a short English version and shared it with you. And I have already translated this English version into German and now I have a "short" German hand out (13 pages including all figures posted here) and a first draft of the article.

 

Yeah, the "fossil hunting trips" are one of my favorite topics of TFF. So much to learn!

 

Thanks again for all your appreciation!
Franz Bernhard