Co-occurrence of Neusticosaurus edwardsii and N. peyeri (Reptilia) in the Lower Meride Limestone (Middle Triassic, Monte San Giorgio)
Summary (2 min read)
Introduction
- The Cassina (also known as ‘‘alla Cascina’’) beds belong to the world-renowned fossiliferous levels of the Middle Triassic Monte San Giorgio Lagerstätte (UNESCO World Heritage List, Canton Ticino, Southern Alps; Fig. 1), particularly famous for its rich and diverse fauna of Middle Triassic marine vertebrates (e.g. Kuhn-Schnyder 1974; Bürgin et al. 1989).
- The aim of the ongoing excavation is to carefully re-document the level in order to provide a better characterization of the basin.
- Besides over three hundred fish specimens, a new pachypleurosaurid (Reptilia, Sauropterygia) specimen of the genus Neusticosaurus was found, which is described and discussed below.
Geological setting
- The Middle Triassic succession at Monte San Giorgio (Figs. 1, 2) starts with a fluvio-deltaic sequence dated to the late Anisian (Bellano Formation, Illyrian; Sommaruga Editorial handling: Jean-Paul Billon-Bruyat.
- The overlying 400–600 m thick Meride Limestone (Furrer 1995) begins with the Lower Meride Limestone, 90 m (Wirz 1945) to 150 m thick (Furrer 1995), which bears three fossil tetrapod beds (Cava inferiore, Cava superiore and Cassina beds), each yielding different vertebrate assemblages (Sander 1989; Bürgin et al. 1989) and consisting of finely laminated limestones with intercalated volcanic ash layers.
- The first diversification of pachypleurosaurids in the western Tethyan realm followed the marine transgression that began in the late Early Triassic and proceeded from east to west across central Europe.
- If correct, the geographic and temporal distribution pattern of N. pusillus described above would support the view of Rieppel and Hagdorn (1997), according to which the Serpianosaurus–Neusticosaurus clade diversified in the southern Alpine intraplatform basin facies and later (N. pusillus) returned to the Germanic basin during the Ladinian via the Burgundy gate.
Materials and methods
- The new excavation site lies to the south of the summit of Monte San Giorgio, a little way out of the outcrop where the Cassina beds were originally discovered in 1933 (Fig. 1).
- So far, the upper third of the succession (Fig. 3) has been excavated bed by bed over a surface of around 40 m2, yielding a well-preserved vertebrate fauna mainly composed of fishes belonging to at least six species, dominated by the large predatory actinopterygian Saurichthys.
- Reptiles turned out to be rare in the upper part of the Cassina beds and the specimen belonging to the genus Neusticosaurus described here is the only articulated find recovered so far.
- The Cassina beds are traditionally regarded as early Ladinian in age (e.g. Hellmann and Lippolt 1981; O’Keefe et al. 1999) but reliable index fossils such as ammonoids and conodonts have never been reported from this horizon.
- This preservation pattern is ascribed to the development of decay gas in the wide abdominal cavity which held the body with the ventral side upward (Furrer 2003).
Systematic palaeontology
- Class Reptilia LAURENTI 1768 Superorder Sauropterygia OWEN 1860 Order Eosauropterygia RIEPPEL 1994 Infraorder Pachypleurosauroidea HUENE 1956 Family Pachypleurosauridae NOPCSA 1928 Genus Neusticosaurus SEELEY 1882 Neusticosaurus peyeri SANDER 1989.
- The squamosal is visible on the right side of the skull, articulated with the small quadrate and quadratojugal by a lateral process.
- Five pairs of caudal ribs can be observed, slightly displaced from their corresponding vertebrae; they show a tapering distal end and their size decreases abruptly after the fourth one.
- The proximal head is thick and subtriangular in section, while the distal head is flattened; the distal articular surface is nearly flat with slightly raised margins.
- In summary, the morphometrical and morphological features described above give, as a whole, reasonable support to the attribution of MCSN 8076 to Neusticosaurus peyeri.
Conclusions
- The attribution of specimen MCSN 8076 to Neusticosaurus peyeri on the basis of both morphological and morphometrical data raises some questions regarding the strict correlation between pachypleurosaurid taxa and fossiliferous horizons in the Monte San Giorgio basin as originally proposed by Sander (1989) and afterwards supported by O’Keefe and Sander (1999).
- Postulating a reversal from N. peyeri to N. edwardsii would no longer be necessary if the two species co-existed, possibly occupying different niches, as the finding of MCSN 8076 in the Cassina beds suggests.
- The authors would also like to thank Heinz Furrer for information concerning the specimen from the Acqua del Ghiffo site and for permission to publish data on its identification.
- The manuscript benefited greatly from the remarks and comments of the reviewers Michael Maisch (Tübingen) and Olivier Rieppel .
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Citations
4 citations
Cites background from "Co-occurrence of Neusticosaurus edw..."
...The latter famously includes reptiles of the Ichthyosauria, Pachypleurosauridae, Placodontia and Protorosauria (Peyer 1944; Kuhn-Schnyder 1974; Furrer 2003; Stockar and Renesto 2011; Furrer and Vandelli 2014)....
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References
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160 citations
"Co-occurrence of Neusticosaurus edw..." refers background in this paper
...…Frauenfelder 1916), is an alternation of black shales and dolomites up to 16 m thick, including in its uppermost part the Anisian–Ladinian boundary (Brack et al. 2005); a volcanic ash layer lying a few metres below this boundary resulted in an U–Pb minimum age of 242.1 ± 0.6 Ma (Mundil et al.…...
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...Furrer et al. (2008) tentatively correlated the Cassina beds with the lowermost Wengen Formation of the GSSP section at Bagolino (Protrachyceras archelaus ammonoid zone, late Ladinian; Brack et al. 2005)....
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155 citations
149 citations
"Co-occurrence of Neusticosaurus edw..." refers background or methods or result in this paper
...…evidence of habitat partitioning, based on the marked differences in adult size (up to 550 mm and up to 1,200 mm in overall length, respectively, according to Rieppel 2000), as well as in dentition (the tooth count is higher in N. edwardsii, which also shows more functional teeth; Sander 1989)....
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...Along with a large collection of specimens attributed to N. peyeri (according to Sander 1989 the only species occurring in this horizon) one specimen was recovered and ascribed to N. edwardsii (MCSN 5624; Heinz Furrer, PIMUZ, personal communication, 2009)....
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...In addition, the dorsal vertebrae count (probably 20) is lower than in N. pusillus (22–24; Sander 1989, p. 578; Rieppel and Lin 1995, p. 35; Rieppel 2000, p. 53)....
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...…(Wirz 1945) to 150 m thick (Furrer 1995), which bears three fossil tetrapod beds (Cava inferiore, Cava superiore and Cassina beds), each yielding different vertebrate assemblages (Sander 1989; Bürgin et al. 1989) and consisting of finely laminated limestones with intercalated volcanic ash layers....
[...]
...Medially, the long, thin splenial is visible, followed by the slim prearticular which lies below a deep surangular; a small coronoid bone seems to be present, as in N. pusillus and N. peyeri (Sander 1989), but no coronoid process is visible....
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Frequently Asked Questions (15)
Q2. How many pairs of caudal ribs can be observed?
Five pairs of caudal ribs can be observed, slightly displaced from their corresponding vertebrae; they show a tapering distal end and their size decreases abruptly after the fourth one.
Q3. What is the composition of the upper meride limestone?
The overlying Upper Meride Limestone is made up of alternating wellbedded limestones and marlstones with an increasing clay content towards the top, where strong seasonal variations of salinity and water level together with the influence from a neighbouring emerged area are documented.
Q4. What is the lateral view of the chevrons of the third to the seventh cau?
The chevrons of the third to the seventh/eighth caudal vertebrae are preserved; they are small elements, showing the distally expanded shape and rounded ventral margin which is typical of neusticosaurs.
Q5. What is the first diversification of pachypleurosaurids in the western?
The first diversification of pachypleurosaurids in the western Tethyan realm followed the marine transgression that began in the late Early Triassic and proceeded from east to west across central Europe.
Q6. What is the morphological character of the wedge-shaped skull?
The wedge-shaped skull with a waisted postorbitalregion which is shorter and wider than the antorbital region rules out N. pusillus, which has a narrow skull table with parallel margins (Carroll and Gaskill 1985; Sander 1989; Rieppel and Lin 1995; Rieppel 2000).
Q7. What is the lateral view of the first 10 caudal vertebrae?
The first 10 caudal vertebrae are also preserved in lateral view (Fig. 7); they show the rib articulations shifting gradually from the neural arch to the centrum, so that, from the fifth caudal vertebra on, the transverse process lies on the centrum only.
Q8. What is the articular surface of the first 10 caudal vertebrae?
They are short, stout and straight, converging on one point to meet the ilium, where they form a well-developed articular surface.
Q9. What is the significance of the co-occurrence of N. peyeri and N. ?
The co-occurence of N. peyeri and N. edwardsii in the same levels implies that the two species indicate possible adaptations toward different size- and/or trophic-related niches rather than representing an anagenetic lineage.
Q10. What is the morphological character of the third carpal in N. edward?
the third carpal in N. edwardsii is a tiny element so that its absence in MCSN 8076 could be due to either a preservation bias or a lack of ossification in case of its being a juvenile specimen.
Q11. What is the earliest known date of the Cassina beds?
The Cassina beds are traditionally regarded as early Ladinian in age (e.g. Hellmann and Lippolt 1981; O’Keefe et al. 1999) but reliable index fossils such as ammonoids and conodonts have never been reported from this horizon.
Q12. What is the palatine shape of the skull?
The palatines are thin and wide, and close the palate completely; they cover the bones of the skull roof, precluding the description of the latter.
Q13. what is the femur/standard length ratio of n. edward?
The femur/standard length ratio of MCSN 8076 is 1.01, well within the range of N. peyeri (0.78–1.06), while the range for N. edwardsii extends from 0.65 to 0.96, averaging 0.73 in juvenile specimens (specimen 3447 in Carroll and Gaskill 1985), therefore indicating a distinctly shorter femur than in MCSN 8076.
Q14. What is the bulk of Neusticosaurus material from the Cassina beds?
According to their examination of the historical collection stored at the Paläontologisches Institut und Museum der Universität Zürich (PIMUZ) and to unpublished field data (both courtesy Heinz Furrer, PIMUZ), the bulk of Neusticosaurus material from the Cassina beds comes from just the base of the sequence, which lies approximately 2 m below the bed bearing the specimen described here.
Q15. What is the preservation pattern of the Neusticosaurus specimen?
This preservation pattern is ascribed to the development of decay gas in the wide abdominal cavity which held the body with the ventral side upward (Furrer 2003).