The Middle Ordovician conodonts Eoplacognathus robustus Bergström and E. lindstroemi (Hamar): Taxonomy and apparatus reconstruction

The Middle-Upper Ordovician Ponón Trehué Formation cropping out in the San Rafael Block (Argentina) provided a large collection of conodonts which have been published years ago. The species Eoplacognathus robustus and Eoplacognathus lindstroemi recovered from these strata have been revised and restudied. In this contribution we describe M and S elements that were assigned previously to Baltoniodus prevariabilis and Baltoniodus variabilis as belonging to the E. robustus and E. lindstroemi apparatuses. The morphological characters of the P, S and M elements as part of these late Eoplacognathus apparatus were analyzed and support the proposal of a new reconstruction of it. We have compared this apparatus with the Baltoniodus apparatus and suggest the inclusion of the Eoplacognathus genus into the Balognathidae family. The new architecture of the Eoplacognathus apparatus is comparable to the genus Lenodus. This analysis improves the knowledge about this index conodont group allowing a new insight on the evolution, phylogeny and paleobiology of this linage.


Introduction
The Eoplacognathus apparatus was first described and their species defined as zonal and subzonal index conodonts by Bergström (1971), supporting this proposal on the P elements.This original description is only based on the oral shape of these P elements.
The key conodonts E. robustus Bergström and E. lindstroemi (Hamar) document the homonymous subzones for the conodont-bearing strata.
The Middle-Upper Ordovician strata of Ponón Trehué outcrops, exposed in the San Rafael Block (Fig. 1), provided a large collection of conodonts.Previous contributions on this conodont fauna were carried out by Heredia (1982Heredia ( , 1998Heredia ( , 2001Heredia ( , 2002) ) and Lehnert et al. (1999).Recently Heredia et al. (2014) and Heredia and Mestre (2015) restudy the conodont collection from the Ponón Trehué Formation presenting a new interpretation of the conodont apparatus of Eoplacognathus.A deeper revision of these materials allow us to assign S and M elements previously assigned to the Baltoniodus species as belonging to the E. robustus and E. lindstroemi apparatuses.
The main purpose of this contribution is to analyze the morphological characters and taxonomy of P elements of Eoplacongathus robustus and E. lindstroemi, and include S and M elements as part of the Eoplacognathus apparatus, thereby proposing a new reconstruction of it; additionally, we compare it to the Baltoniodus apparatus.

Geological setting
Within the Ponón Trehué region is located the only preserved record in all Cuyania of the Lower Paleozoic carbonate-siliciclastic succession in depositional contact with basement rocks.This basement is interpreted to be of Grenvillian (late Mesoproterozoic) age (Ramos, Vujovich and Dallmeyer, 1996;Cingolani and Varela, 1999) and constitutes the Cerro La Ventana Formation.
The outcrops in the Ponón Trehué area allow for the recognition of three different units.One is composed entirely of the Mesoproterozoic basement.The other two are sedimentary units that include varying amounts of reworked basement rocks.A well-established Ordovician clastic succession of the Ponón Trehué Formation (upper Darriwilian to lower Sandbian) is composed of granite conglomerate, sandstone and thin-bedded, fossiliferous limestone.A significant unconformity can be traced between the Ordovician clastic sequence and the underlying basement, exposed to the east.The dominant features of these Mesoproterozoic and Ordovician rocks are their isolate, discontinue and disperse outcrops in a green shale matrix suggesting an olisthostromic origin for these deposits (Heredia and Mestre, 2017).Astini (2002) considered that the limestone outcrops of the Ponón Trehué Formation (sensu Bordonaro, Keller and Lehnert, 1996) were blocks and fragmentary carbonate bodies discontinuously exposed, floating in arkose conglomerate, in agreement with Heredia (1998,2001), Beresi and Heredia (2000) and Heredia and Mestre (2017).In spite of these interpretations these deposits are informative about their fossil record and history.The Ordovician outcrops in Ponón Trehué represent a depositional cycle from shallower to deeper environments.This succession involves two different deposits: the lower one comprises coarse siliciclastic deposits and the upper one consists of fine, dark carbonate-fine clastic deposits.The biostratigraphy of these Ordovician outcrops has been based on conodonts, recognizing two biozones: the Pygodus serra Zone and Pygodus anserinus Zone (Heredia, 1982;Bordonaro, Keller and Lehnert, 1996;Heredia, 1996;Lehnert et al., 1999;Heredia, 2001;Cingolani and Heredia, 2001;Heredia and Mestre, 2017) (Fig. 2).Brachiopods, sponge spicules (Beresi and Heredia, 2000), bryozoans, ostracodes, crinoids, trilobites and algae fragments were also recovered from the analyzed samples.
These are well known from numerous studies of abundant material throughout Middle Ordovician sections in Baltoscandia, North America, Argentina and China.
Dzik (1976) considered that the evolution process of the Eoplacognathus species let only platform elements (Pa and Pb) in the apparatus.Later, Dzik (1994) described a multielemental apparatus for the earliest species of the genus Eoplacognathus: E. zgierzensis Dzik, conformed by Pa, Pb, S and M elements.Stouge and Bagnoli (1990) reported for the first time S and M elements in the apparatus of Eoplacognathus pseudoplanus proposing a change in this genus name to Lenodus pseudoplanus, including the genus Lenodus to the Balognathidae Family.Stouge and Bagnoli (1999) kept the genus Eoplacognathus in the Polyplacognathidae.However, Löfgren and Zhang (2003) continue to use the designation Eoplacognathus pseudoplanus, and they recognized S and M elements for this species.Melgren et al. (2012) recognized S elements in the E. foliaceous apparatus.
A detailed review of the P elements basal cavity from the Eoplacognathus species showed that is different in earlier representatives comparing with later ones, been the first ones scaphate (E.pseudoplanus to E. robustus species) and the later ones are planate (E.lindstroemi and E. elongatus species).This character appeared earlier in the E. robustus sinistral Pa element.We propose that this feature can be used as tool for species recognition between E. robustus and E. lindstroemi.
Zhang (1998) studied the Polyplacognathidae family proposing changes in its interpretation, recognizing two new genera Baltoplacognathus and Yangtzeplacognathus, together with Eoplacognathus-Polyplacognathus and Cahabagnathus represent four evolutionary lineages in the Middle Ordovician.The index species Yangtzeplacognathus foliaceous, Baltoplacognathus reclinatus, B. robustus, E. lindstroemi and E. elongatus were redefined among others.Zhang (1998) supported this proposal based on differences on the morphological features of the Pa elements (Baltoplacognathus) or Pb elements (Yangtzeplacognathus), linking them discontinuously.Especially the differences between both genera are expressed mainly in the length and width of the processes and angles between them, whereas the differences between species of the first genus lie on the denticles row of the processes (Zhang, 1998).
Several elements identified as E. robustus-E.lindstroemi transitional forms were recovered from the PT10 sample, in the Ponón Trehué section (Heredia, 2002).These types of elements are defined as a transitional form due to the features that exhibit such as size and shape of the processes and lobes.There is a trend that can be followed in the Pa element morphology of the species E. robustus-E.robustus/E.lindstroemi transition-E.lindstroemi.Based on this evidence, we reject the use of the genus Baltoplacognathus for the species robustus.
The basal cavity of the Eoplacognathus P elements was shortly described by Bergström (1971).He mentioned that the basal cavity is scaphate in juvenile specimens and planate in mature specimens for the most of species of this genus, except E. lindstroemi and E. elongatus.These descriptions do not reflect the true morphology of the basal cavity, after that we consider the necessity of taxonomical revision of this genus.Descriptions of the P element basal cavities are herein developed, it is worth noting that they are different in the early E. robustus species when compared to the descendent E. lindstroemi, being the first one scaphate and the later one are planate.We propose that this feature could be used as an additional tool for diagnosing between E. robustus and E. lindstroemi.

Taxonomy
The synonymy lists are condensed, mostly containing only the original citations of species names incorporated in each multielement taxon.In the descriptions, we have used the conventional orientational terms -anterior, posterior, and lateralnoting these do not relate to the anatomical orientation of elements (see Purnell et al., 2000).
The S elements location in the Baltoniodus apparatus follows the Stouge and Bagnoli (1999) interpretation of it.
M. The M element is represented by a modified geniculate morphology.The cusp is erect and has a central carina on the inner side, and is longer than the anterior margin.The angle between anterior margin of the cusp and the anterior margin of the base is greater than 180°.The angle between anterior and oral margins is less than 90° and has a U-like shape.
Sa.The Sa element has an alate morphology.The element has three regularly denticulate processes.The denticles are fused in the base and climb through the cusp.
Sb.The tertiopedate element, clearly asymmetrical.The cusp is short, erect to the tip.The three processes have regular proclined denticles toward the cusp.
Sd.The symmetrical quadriramate elements has a short cusp and all processes are regularly denticulate and they are connecting with the cusp through a carina.
Eoplacognathus lindstroemi (Hamar, 1964) Fig. 4, A-G Materials.Four hundred eighty five elements.Three hundred Pa, one hundred seventy three Pb, eight S, and four M elements.Description.We have nothing to add to the description by Bergström (1971) on the Pa and Pb elements.
M. The M element is represented by modified geniculate morphology.The cusp is erect and is longer than the anterior margin.The angle between anterior margin of the cusp and the anterior margin of the base is greater than 180º and the anterior margin is thin and short.The angle between anterior and oral margins is less than 90° and has a U-like shape.
Sa.The Sa element has an alate morphology.The element has three regularly denticulate processes.The denticles are fused in the base and climb through the cusp.
Sb.The tertiopedate element, clearly asymmetrical with a short cusp and erect to the tip.The three processes have regular and similar in shape procline denticles.These denticles appear high on the cusp.
Sd.The symmetrical quadriramate element with a short cusp.All the processes are regularly denticulate and connecting with the cusp through a carina.
Sd and Sa element present bad preservation, for this reason, we did not illustrate these both element.

Remarks
The Eoplacognathus apparatuses herein described are composed by six elements: Pa, Pb, M, Sa, Sb, and Sd.The P elements are abundant but the S and M elements are very poor in number.The recovering of S and M elements is possible when the collection of Eoplacognathus is large.The absence of the Sc element and decreasing number of the other S and the M elements could be explained following Löfgren and Zhang (2003) observation; these authors suggested a decrease of S elements in evolutionary trends between Lenodus-Yangzeplacognathus-Eoplacognathus apparatuses and the progressive disappearance of the bipennate element into these apparatuses.The Eoplacognathus robustus and E. lindstroemi apparatuses from Ponón Trehué were compared to Eoplacognathus pseudoplanus and E. suecicus apparatuses from the Precordillera (Heredia, 2012;Mestre, 2012;Mestre and Heredia, 2013) (Table 1, Fig. 5).The analysis of these data inform that there is a great increasing of P elements over the S and M elements, and the early representatives of the genus neither provided Sc elements.
We have recognized that the S and M elements from E. robustus and E. lindstroemi can be identified as belonging to mature specimens.This datum allows inferring that the S and M elements representing juvenile and adult specimens are missing.Dzik (1976) proposed that the evolutionary process of the Eoplacognathus species let only platform elements in the apparatuses by the progressive demineralization of the S and M elements.Perhaps the S and M mature elements from our collection are mineralized as part of the ontogenetic process.
The S and M elements of the E. robustus and E. lindstroemi (early and late form) apparatuses were compared with those of Baltoniodus prevariabilis and B. variabilis (Fig. 6).The main differences in the S elements of these two genera are the orientation, shape, distribution and height of the denticles and the absence/presence of micro-striation on the cusps.

FIG. 2 .
FIG. 2. Stratigraphic column in La Tortuga section with the ranges of the selected conodont species and conodont biostratigraphical scheme for the Ponón Trehué Formation (Modified from Heredia and Mestre, 2017).

TABLE 1 . COMPARATIVE TABLE OF EOPLACOGNATHUS
(Eoplacognathus lindstroemi)from the Precordillera and Ponón Trehué, respectively; amount and percentage of elements from each species displayed on figure 5.