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AMEGHINIANA (Rev. Asoc. Paleontol. Argent.) - 47 (4): 447-459. Buenos Aires, 30-12-2010 ISSN 0002-7014 ©Asociación Paleontológica Argentina AMGHB2-0002-7014/10$00.00+.50
1División Paleontología Vertebrados,
Museo de La Plata,
Universidad Nacional de La Plata - CONICET.
Paseo del bosque s/n, 1900 La Plata, Argentina.
2Centro de Investigaciones Geológicas,
Universidad Nacional de La Plata - CONICET,
calle 1 Nº 644, 1900 La Plata, Argentina.
The oldest lower Upper Cretaceous plesiosaurs (Reptilia, Sauropterygia) from southern Patagonia, ArgentinaJosé Patricio O’GORMAN1 and Augusto Nicolás VARELA2
Key words. Plesiosaurs. Mata Amarilla Formation. Upper Cretaceous. Santa Cruz. Argentina. Palabras clave. Plesiosaurios. Formación Mata Amarilla. Cretácico Superior. Santa Cruz. Argentina.
Abstract. Plesiosaurs are recorded for the first time from the lower section of Mata Amarilla Formation, Santa Cruz Province, Patagonia, Argentina. The stratigraphic succession consists of mudstones and siltstones interbedded with medium to fine-grained sandstone, deposited in a littoral environment during the Cenomanian-Santonian; therefore the material is the oldest record of plesiosaurs from the lower Late Cretaceous rocks of Argentina. The remains include teeth, some vertebrae, and one propodium assigned to Elasmosauridae indet. and Plesiosauria indet. The status of Polyptychodon patagonicus Ameghino, 1893, as well as its stratigraphic position are discussed, leading to the conclusion that the material described by Ameghino is probably from the Mata Amarilla Formation and can only be referred to Plesiosauria indet. Analysis of sedimentologic features suggests that the material described here was deposited in an estuarine environment, strongly influenced by tides. The characters of the inferred environment are consistent with the type of preservation of the materials.
'Plesiosauria indet.' tooth from Patagonia
Comments on Polyptychodon patagonicus Ameghino, 1893
Polyptychodon patagonicus was described by Ameghino in 1893. The type material of Polyptychodon patagonicus consists of a group of teeth -described but never illustrated- coming from what Ameghino called and understood as “formación Santacruceña” in 1893. Ever since the 19th century there have been different opinions about the age and correct assignment of this material. The discovery of plesiosaur vertebrae and teeth in the lower section of the Mata Amarilla Formation leads to the analysis of the history of Florentino Ameghino’s ideas on the age of the layers bearing the remains, and how this influenced the assignation of the materials to other reptile taxa. Ameghino made the following description of the original materials of Polyptychodon patagonicus:
[The teeth of Polyptychodon patagonicus are open at their base, conical-cylindrical in shape, with sharp-pointed apex, and strongly curved. The outer surface is covered by pronounced longitudinal ridges of enamel, separated by deep furrows; the ridges start at the base, all of them at the same level, ending at different distances from the apex, nearly in the same way as in Polyptychodon interruptus, Owen. I do not know any complete tooth. The largest tooth, which almost lacks the entire base occupied by the pulp cavity, is nearly 3 cm high; and at the broken part at the base, it measures one centimeter in diameter approximately] (Ameghino, 1893, p. 82; translated from Spanish).
However, subsequent authors considered Ameghino ´s identification (1893) with doubts, probably because of two reasons. First, plesiosaur teeth have been often confused with crocodile or fish teeth, because of their similar morphological characteristics. Second, the association of Polyptychodon patagonicus teeth with Cenozoic mammalian teeth in the same formation was assumed. For this reason Cabrera (1941) stated that if the teeth of Polyptychodon patagonicus were contemporaneous with mammalian remains, then they should be assigned to crocodilians. Welles (1962) followed the same line of thought and mentioned Cabrera (1941), claiming that the teeth could belong to crocodilians. Finally, in a summary of South American plesiosaurs, Gasparini and Goñi (1985) mentioned Polyptychodon patagonicus as a plesiosaur, but with serious doubts about its assignation. Unfortunately, the original material described by Ameghino in 1893 is not available for study. Although there is a record of two teeth of Polyptychodon patagonicus from the “Sub-Patagoniano” of Lago Argentino in the MACN, this material (catalogued as MACN-A 5809) could not be located in the collection.
Concerning the stratigraphic and geographic provenance of his material, Ameghino (1893, p. 76) wrote: [The formation that I have named Santacruceña, covers most of the Austral Patagonian region that is crossed by the Santa Cruz, Sehuen, and Gallegos Rivers] (Ameghino, 1893, p. 76; translated from Spanish). It is clearly evident that the “formación Santacruceña” named by Ameghino, is equivalent only in part to what is nowadays known as the Santa Cruz Formation, as the Santa Cruz Formation is not exposed along the Shehuen or Chalia River (Sehuen River of Ameghino). Contrarily, the Mata Amarilla Formation is the unit exposed along the mentioned river (Feruglio, in Fossa Mancini et al., 1938; Arbe, 1989, 2002; Varela and Poiré, 2008). For this reason it is evident why Ameghino included the Mata Amarilla Formation in his “formación Santacruceña” (Cione et al., 2007). Ameghino’s opinion is quite understandable taking into account the lithological similarity of the Mata Amarilla and the Santa Cruz formations.
In relation to the age, Ameghino (1893, p. 76) assigned his “formación Santacruceña” to the “Eoceno inferior (Paleoceno)”. Due to the problematic association of primates and other Cenozoic mammals with Cretaceous taxa in the “formación Santacruceña”, Ameghino wrote in the final discussion:
[This fact (referring to the identifications carried out in the paper and the associated Cretaceous age) is meaningful and enough to decidedly tilt the scales in favor of those who believe that the “formación Santacruceña” is even older than the “lower Eocene”, as I considered it from the beginning. It is possible that it becomes necessary to refer the lower part of it to the larámico or upper Cretaceous] (Ameghino, 1893, p. 84; translated from Spanish).
So what seemed to be an association between Polyptychodon patagonicus and mammalian teeth was [possibly] just a lithostratigraphic misunderstanding of Ameghino, who rectified himself in 1906. It is important to emphasize that the original purpose of the 1893 paper was not to solve the stratigraphical problem, but to add new descriptions of the material sent by his brother, Carlos Ameghino. However, since 1893 Ameghino supposed that his “formación Santacruceña” included units of different age. In his work published in 1906, Ameghino separated the outcrops at Río Shehuen initially belonging to the “formación Santacruceña”, and named them “Sehuenense stage”. Ever since then Polyptychodon patagonicus was listed together with taxa from the “Sehuenense stage” instead of being included with those from the “formación Santacruceña”, thus solving the stratigraphic problem. Hence, it can be assumed that there is no real association between Polyptychodon patagonicus and Cenozoic taxa. It is necessary to point out that different authors refer to the “Sehuenense” in different ways. [Emphasis added-DD] Ameghino (1906) used “Sehuenense”, while Cabrera (1941) used “Sehuense”, and Arbe (1989, 2002) “Shehuenense”. We use the first spelling in this contribution because we refer to Ameghino’s original concept (1906). Under the light of the discussion above, and with the new record showing the existence of abundant dental material from the Mata Amarilla Formation assignable to Plesiosauria, there appears to be strong evidence that Ameghino was the first to publish material referred to Plesiosauria from lower Late Cretaceous rocks in Santa Cruz.
Since the original material is not available, the relation between the teeth referred to Polyptychodon patagonicus and the new records described here can only be discussed based on Ameghino´s description and the diagnosis of the genus Polyptychodon Owen, 1841 Tooth morphology described by Ameghino (1893) is common in plesiosaurs: conical, slightly curved teeth, with marked striae. Polyptychodon teeth illustrated by Owen in 1841 seem to bear striae on the entire outer surface The fact that Ameghino never mentioned any difference between his material and the teeth illustrated by Owen could indicate that his material had the same features shown by Polyptychodon. Therefore, the presence of striae on the entire outer surface is a difference between the original material of Polyptychodon patagonicus and the materials described in this paper, which have reduced or even absent lingual striation. The reasons stated by other authors (Cabrera, 1941; Welles, 1962) to reject the identification of the teeth made by Ameghino are thus not valid. The assignment of Ameghinos’s material to Polyptychodon, a pliosaur from the Upper Cretaceous (Cenomanian- Campanian?) in England, Germany, Czech Republic, and USA (Welles and Slaughter, 1963, Bardet and Godefroint, 1995) is not valid because no characters support this assignation. Hence, Polyptychodon patagonicus is a nomen vanum and the type material, as well as the teeth described in this paper, must be referred to Plesiosauria indet. [Indefinite Plesiosauruia]
Paleoenvironment interpretation The lower section of the Mata Amarilla is well exposed in LB section, where the plesiosaur remains were found in white sandstone facies with herringbone cross-stratification, showing changes in the direction of palaeocurrents; according to Boyd et al. (2006) this is interpreted as produced by tidal action. So, following the criteria of Boyd et al. (2006), these facies were interpreted as subtidal bars characterizing the shallowest part of estuaries. Finally, this lower section ends with a prograding bayhead delta. In the MAT section, the lower section of the Mata Amarilla Formation exposes only five meters (figure 3). The skeletal remains of plesiosaurs in the MAT section were found in heterolithic, mixed stratified (flaser and wavy) facies, showing alternating decantation and traction processes. Such structures are mostly dominated by shales, characteristic of estuarine central areas, according to Boyd et al. (2006). The paleoenvironment of the lower section of the Mata Amarilla Formation where the plesiosaurs were found is therefore interpreted as a tidal dominated estuary. All the plesiosaur bones and teeth show signs of transport. The material is disarticulated with no evidence that they belonged to the same single specimen. It is interesting to note that the remains are predominantly elements resistant to fragmentation, such as the vertebral centra, the propodium, and teeth. However, the teeth are incomplete, most without roots and some lacking the apex of the crown. Bone damage consists of fractures and elimination of superficial bone but no perforations or fouling were observed. Therefore, the preservation status of the skeletal remains is consistent with the energetic conditions of tidal action, which is consistent with the sedimentological interpretation. In vertebra MPM-PV 1871-2 (Level 2, LB section), the internal cavity is filled with sediment forming a natural cast of the missing part of the vertebra. Clearly, the fracture occurred after lithification, suggesting that part of the damage observed in the material could have been caused by reworking or recent weathering. Finally, although very scarce, some remains of the lower Mata Amarilla Formation are young specimens, possibly lying near the coast, probably an estuary. It has been often mentioned that young specimens, at least those of plesiosauroids, could live in rivers and estuaries (Wiffen et al., 1995).
Biogeographic significance of the new record Late Cretaceous plesiosur records in southern Gondwana were limited to the Campanian-Maastrichthian of Argentina (Cabrera, 1941; Gasparini and Goñi, 1985, Gasparini and Salgado, 2000; Gasparini et al., 2003a,b; 2007), Chile (Gay, 1849; Casamiquela, 1969; Suárez and Fritis, 2002), Antarctic Peninsula (Gasparini et al., 1984; Chatterjee and Small, 1989; Fostowicz-Frelik and Gazdzicki, 2001; Martin et al., 2007), New Zealand (Wiffen and Moisley, 1986; Cruickshank and Fordyce, 2002; Hiller et al., 2005), and the Turonian-Cenomanian of Australia (Kear, 2003). According to those records it is evident that while the early Late Cretaceous is recorded in Oceania, there are no records for the same period in southern South America and Antarctica. The new plesiosaurian material from the lower section of the Mata Amarilla Formation contributes toward completing this gap in the record (figure 9).
ConclusionThe oldest lower Late Cretaceous plesiosaur record in Argentina is presented, becoming the first record of these reptiles for Santa Cruz Province. Analysis of the literature provides strong evidence that Polyptychodon patagonicus Ameghino, 1893, is based on remains of Plesiosauria from the Mata Amarilla Formation. However, the status of nomen vanum is maintained for P. patagonicus because the isolated teeth are not considered sufficient to diagnose a genus or species. The lower section of the Mata Amarilla Formation in the study area was deposited in an estuarine environment with strong tidal influence. The conservation and lack of articulation of the remains is congruent with the expected level of transport in that environment.
Acknowledgments The authors thanks T. Martin (Institut für Palaeontologie, Universität Bonn), F. Goin and J. Gelfo (Museo de La Plata) for providing logistical support in field work, in which also participated J. Cuitiño, G. Pedersen, A. Barrueco, M. Giacobelli, L. Chornogubsky, S. Hoffmann, and J. Schultz. They also thank Dr. Adan Tauber (Museo Regional Provincial Padre Manuel Jesús Molina, Río Gallegos) for authorization to study the material from Santa Cruz, and L. Acosta Burllaile for the preparation of material. C. Deschamps and J. Echevarria (Museo de La Plata) for reading the English version. Likewise they tanks their dissertation advisors, Z. Brandoni de Gasparini, L. Salgado, and D. Poiré for critically reading of the manuscript and for financial support (projects PICT 25276 and CONICET PIP 6237/05). Finally they thank the referees, N. Bardet (Muséum national d´ Histoire naturelle, Paris) and J. Martin (Museum of Geology, South Dakota School of Mines and Technology, USA), whose comments have substantially improved both the scientific content and the language of this paper.
-Basically, the record stands as a situation where Ameghino, the original author, named some Plesiosaur teeth as coming from a stratum which he believed to be Cenozoic and after the extinction of the dinosaurs: However, he was using the name for the stratum that different authors treat differently and which could be interpreted as coming from lower down in the Cretaceous. The authors of this article assume that this must have been the case but that is not necessarily so. Several subsequent Plesiosaur finds from South America have been reported as coming from ambiguous or Post-Cretaceous strata. Although the matter has been conveniently explained away, the uncertainty actually still remains and some of the remains could actually be from the post-Cretaceous and associated with early Cenozoic mammals , as was actually originally reported.-DD]