LON-CAPA alb21-3

A DEBATE ON THE LADINIAN-CARNIAN BOUNDARY

Paolo Mietto and Stefano Manfrin

Introduction

Research on the ammonoid distribution in the Ladinian-Carnian boundary interval in the Southern Alps (Italy) allowed Mietto & Manfrin (1995a, b) to define a stratigraphical interval - characterized by Daxatina, Clionitites and by some species of Trachyceras different from T. aon - below the traditional base of the Carnian (= base of the Aon Zone/Subzone). The interval, defined in the Prati di Stuores section (Pralongià, Dolomites), was named the Daxatina cf. canadensis Subzone. As a consequence of the new biostratigraphical data, it was suggested that the base of the Carnian be placed in a lower stratigraphical level. Therefore the upper part of the Frankites regoledanus Zone (sensu Krystyn in Zapfe, 1983), usually considered Ladinian and correlated with the upper Frankites sutherlandi Zone in North America, should be included in the Carnian.

The proposal aroused interest in a group of Italian researchers (Loriga Broglio & Neri, 1995) who studied other groups of fossils, as well as the magnetostratigraphy and the sequence stratigra phy of the Prati di Stuores section, surely the most interesting and complete in the Dolomites among those including the L/C boundary.

The suggestions, one of lowering the base of the Carnian Stage using the first appearance of the cosmopolitan genus Daxatina as a criterion for recognizing it (Mietto & Manfrin, 1995a,b), and the other of considering the Prati di Stuores section as the Global Stratotype Section and Point (GSSP) for the base of the Carnian Stage, were made in Broglio Loriga et al. (1998b) and formally proposed in Broglio Loriga et al. (in press). In a meeting at Pralongià, 2-3 July 1998, the stratigraphic section of Prati di Stuores was visited by the members of the S.T.S. and the research was presented and discussed. An internal report (Broglio Loriga et al., 1998a) was also prepared.

Later Balini and others (1998) reported the results of their research at Spiti (Himalaya) on the same interval, with interesting data on conodonts and pelagic bivalves, not well documented at Stuores. The writers doubt some conclusions of Balini and et al. (1998), and will discuss them.

Remarks Balini et al. (1998) present two kinds of problems that will be discussed and analysed. The first kind is formal - in Balini et al.'s opinion as well - and is related to taxonomy. The second one is more significant and concern methodological and philosophical concepts.
Taxonomy When the suture line is not visible, Daxatina is not easily distinguished from Trachyceras; this is well illustrated in Tozer (1994), for example. The criticism about the Trachyceras muensteri illustrated in Mietto & Manfrin (1995a: pl. 5, fig. 5; 1995b: pl. 2, fig. 16) and related as Daxatina cf. canadensis in the internal report (Broglio Loriga et al., 1998a) is clearly a mistake, from which Balini et al. (1998) conclude that at Stuores "...the ammonoid taxonomy does not seem to be stable". No experienced specialist in ammonoid taxonomy can mistake T. muensteri with D. canadensis, because of their great morphological differences, and therefore the statement of Balini et al. is clearly unfair.

The specimen in discussion has traces of a clearly Trachyceras-like suture line; many other cospecific specimens were found in beds both over and underlying level PSR3, the probable origin stratum of the above mentioned specimen of T. muensteri, within a 40 cm thick set of layers particularly rich in ammonoids. This is the meaning of PSR3.dt.1 of the quoted specimen. Very good specimens of the same species were found with T. bipunctatum in other coeval stratigraphic sections in the Dolomites, for example in the sections of Campolongo Pass (Sella group).

Another problem is the taxonomical separation between Frankites regoledanus and F. apertus. According to Mietto & Manfrin (1995a, b) the turnover of these taxa is correlatable with the bioevents documented (Tozer, 1994) in Canada:

appearance of Frankites, with F. glaber, at the base of the Sutherlandi Zone (Subzone 1)
replacement of F. glaber by F. sutherlandi at the base of Subzone 2
coexistence of F. sutherlandi and Daxatina canadensis in the middle/upper part of Subzone 2.

The recognition of the same trend among the representatives of Frankites documented in the Southern Alps makes the discrimination between F. regoledanus and F. apertus, doubted in Balini et al. (1998), particularly important. The problem was discussed at the S.T.S. meeting at Pralogià. A first conclusion was that the species named F. apertus by Mietto & Manfrin (1995a, b) can be discriminated from F. regoledanus, first of all for the frequency of the ribs. There was no conclusion as to the name of the species that, according to L. Krystyn (personal communica tion) should be probably considered nova species. From this point of view the F. regoledanus-F. apertus problem is just a nomenclature problem, with no influence on the meaning of the bioevents of the interval and on the possible correlations with the successions in other paleobio geographical provinces. Surely this is an open problem, but it will never ".... lead to serious confusion in the future stratigraphic scale" as concluded by Balini et al. (1998: p. 31).

Another problem posed by Balini et al. (1998) is that the Daxatina "...forerunner is unknown". In our opinion, the forerunner is in the Asklepioceras-Muensterites lineage (cf. Mietto & Manfrin, 1995b: pl. 2, fig. 9). These genera are well known both in low-intermediate latitude domains.

Methodology and "philosophy" If the suture line is not visible, it is difficult to discriminate Daxatina from Trachyceras, so that the former genus "... does not completely fulfil the requirements of a guide fossil" in Balini et al. (1998: p. 30) opinion. If the transitive law is applied, also Trachyceras "... does not completely fulfil the requirements of a guide fossil". If the statement of Balini et al. (1998) is accepted, many taxa should be eliminated as biostratigraphical markers. The problem will occur for example with Schreyerites-Paraceratites or with Eoprotrachyceras-Protrachyceras, involving in the discussion the Pelsonian-Illyrian and the Fassanian-Longobardian boundaries.

In a case similar to that in discussion, Metapolygnathus vel Gondolella polygnathiformis is considered the possible conodont marker for the Ladinian-Carnian boundary. As reported in Broglio Loriga et al. (1998b: p. 9), the species was identified in the Fyred Limestone in the Balaton Highlands by Kovacs et al. (1991), but in an upper Ladinian context, and later denied by one of the authors. In this case, should the problem of the "taxonomical stability" prevent the use of this taxon for biostratigraphical purposes? Of course this has no real meaning. Any taxon, when its distinctive characters are not well preserved, can be valued subjectively; this is a problem of palaeontology and it must be taken into account when the data are used for biostratigraphical purposes, so that the objective and the subjective data are distinguishable. Surely, it is not scientifically correct to refer to non-illustrated taxa to support one's ideas or to discuss others'. Most of the researchers of ammonoid Triassic biostratigraphy, the writers included, have incurred in this mistake. But it is unfair to attribute the mistake to other, without seeing one's own. Balini et al. (1998), for example, point out - as for the ammonoids at Stuores - "... at present the features of these species are unknown ...". The most significant species were at least illustrated in Mietto & Manfrin (1995a, b) and now in Broglio Loriga et al. (in press), although surely the complete taxonomy of the ammonoids found at Stuores must be given in a near future. That being stated, Balini et al. (1998) write "As regard the appearance of Clionitites, in Epidaurus section this genus occurs already at the base of the Regoledanus Zone (sample A16 in Krystyn 1983, fig.3 p. 244) then Mietto & Manfrin's sections record a F.O., not the F.A.D. of the genus". But the "Krystyn 1983, fig.3 p. 244" is only a stratigraphic column of Epidaurus, with no reference to Clionitites. Possibly Balini et al. (1998) referred to Krystyn & Mariolakos (1975), where this genus is reported at Epidaurus associated with F. regoledanus. Anyway, there are no descriptions and/or pictures of these presumed Clionitites. Therefore, at the moment the statement that Clionitites appears before Daxatina and/or Trachyceras is a still undemonstrated opinion. On the basis of the data found in the Southern Alps and of those in the literature, the presumed pre-Carnian Clionitites should be carefully compared to the very similar taxa "Anolcites" cf. laricus and Zestoceras. The latter genus, originally defined in Canada (Tozer, 1994), is now well documented in the Southern Alps, with a set of species comparable - at least as a trend - with what observed in Canada in the Upper Ladinian-Lower Carnian interval (Maclearni to Sutherlandi Zones), as reported in Broglio Loriga et al. (1998a, in press).

Balini et al. (1998) report a possible specimen of Daxatina at level 97/176 of Muth (Spiti). During the S.T.S. meeting at Pralongià, L. Krystyn illustrated the composite Spiti section; the report of Daxatina in Himalaya and his statement about the sure occurrence of the genus in the Tethys domain as well as in the Southern Alps were considered very interesting. The doubt later expressed in Balini and others (1998) about the occurrence of Daxatina at Spiti is very sur prising. Clearly there were the same problems of "taxonomical stability" that disturb Balini et al. when they value other researchers' works.

Synchroneity A very important problem is stressed in Balini et al. (1998) when they doubt the synchroneity of the F.O. of Daxatina in the different paleobioprovinces, and therefore the possibility of recognizing its F.A.D. The occurrence of the cosmopolitan genus Daxatina (the only one in this critical interval) would not be appropriate as the marker of the base of the Carnian Stage. The doubts on the real pre-dating of Trachyceras would make the use of the F.O. of Trachyceras, that in North America is at the base of the Desatoyense Zone, inappropriate as well. In other words, Balini and others seem to support the maintenance of the present criterion, that is the base of the Aon Zone (correlated with the base of the Desatoyense Zone) as the L/C boundary.

As they are the main problem in the correlations among different paleobiogeographic provinces, surely these doubts can be shared on philosophical basis. In the case in exam both the historical correlations and the new proposal can only be based on biostratigraphy, since no comparable data of physical stratigraphy are available at the moment. This stated, at present (cf. Dagys & Tozer, 1989; Dagys & Weitschat, 1993) the base of the Carnian Stage is correlated through the F.O. of Trachyceras (= T. aon) in the Aon Suzone (low latitude: Tethys), the F.O. of Trachycerras (= T. desatoyense) and the occurrence of Stolleyites in the Desatoyense Zone (middle latitude: Canada), and the F.O. of Stolleyites in the Tenuis Zone (high latitude: Arctic Ocean). None of them can be considered a FAD. In the Southern Alps the genus Trachyceras occurs a long time before T. aon; in Canada, Stolleyites occurs in the Desatoyense Zone but seems to be already present in the Ladinian (Tozer, 1994); Trachyceras is not documented in the Boreal domain.

Daxatina appears below the base of the Aon Subzone (Southern Alps, ?Spiti), of the Desatoyense (British Columbia, Nevada) and the Tenuis Zones (Svalbard). The comparison between the ammonoid successions in the Southern Alps and in British Columbia shows an exceptional analogy on taxonomical basis and proves that the bioevents in this interval are basically in homotaxis. On the basis of these valuation elements, the problem of the synchroneity of the F.O. of Daxatina as proposed in Balini et al. (1998) is merely academic.

Now the scientific community has to answer the question, whether it is preferable to refer to an asynchronous and non-cosmopolite event, like the F.Os of species of Trachyceras, non- cospecific in the different domains, or to the F.O. (but why not the F.A.D.?) of the synchronous and sure cosmopolitan genus Daxatina, probably represented by the same species in the various paleobioprovinces.

A basic issue
Up to now, assuming that the F.O. of Trachyceras coincide with those of T. aon and T. desatoyense, every association including Trachyceras was considered lower Carnian (lower Julian) in age. Even if the proposal of lowering the L/C boundary is accepted, the assumption is still basically correct, since Trachyceras is pre-dated. On the basis of the new statement, the associations should be divided into pre- or post-Aon vel Desatoyense biozones.

The precise analysis of the Canadian fossiliferous localities reported in Tozer (1994) allows the identification of a stock of specimens (characterized in particular by Trachyceras? sp., Clionitites callazonensis, C. reesidei, Daxatina limpida and Stolleyites sp. indet.) always found below the first occurrence of T. desatoyense. They are attributed to the Desatoyense Zone, but we think that this is not the only possible interpretation. Moreover, in the Southern Alps the L.O. of Daxatina is older than the base of the Aon Subzone.

As for Trachyceras? sp. reported by Tozer (1994: pl. 88, fig. 14) in the Callazon Creek (British Columbia, GSC loc. 84268), the specimen, the suture line of which is not visible, was found 17 m below the first T. desatoyense (GSC loc. 84269). For the features of the venter it may be a real Trachyceras. The stratigraphic position of the specimen is no guaranty of its sure belonging to the Desatoyense Zone and consequently it cannot be surely stated that in Canada the genus Trachyceras first appears at the base of the Desatoyense Zone. This is true also if Tozer's doubts on the identification of the specimen are accepted.

In this case, taxonomic doubts and different stratigraphic considerations justify subjective interpretations; of course the different interpretations should stimulate further researches.

As a conclusion, it must be stressed once again that the lack of a clear discrimination between objective and subjective data, that is between checkable data (key data) and interpretations, were, are and will always be detrimental for further discussion.

References

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© ALBERTIANA, February 1999