Rediscovery of the type localities of the Late Cretaceous Mongolian sauropods Nemegtosaurus mongoliensis and Opisthocoelicaudia skarzynskii: Stratigraphic and taxonomic implications

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Highlights

  • Relocation of quarries for two sauropod holotypes

  • Description of new material of Nemegtosaurus

  • Discussion of probability that Opisthocoelicaudia is a junior synonym of Nemegtosaurus

  • New data showing sauropods more prevalent in Nemegt Formation ecosystem than previously thought

Abstract

In 1965, the Polish-Mongolian Palaeontological Expeditions recovered two sauropods from the Nemegt Formation of the Nemegt Basin, Mongolia (Kielan-Jaworowska and Dovchin 1968). One specimen, a nicely preserved, complete skull that in 1971 became the holotype of Nemegtosaurus mongoliensis, was found in Central Sayr at the Nemegt Locality. The other was found at Altan Uul IV and is a nearly complete postcranial skeleton lacking only the skull and neck. In 1977, this skeleton became the holotype of Opisthocoelicaudia skarzynskii. Nemegtosaurus and Opisthocoelicaudia were initially assigned to different sauropod higher taxa, Dicraeosaurinae and Camarasauridae respectively. However, since the late 1990s, both genera have been recognized as members of Titanosauria. Their coincident spatiotemporal distribution and non-overlapping skeletal parts have led to the persistent suspicion that they belong to the same species. Rediscovery of the original quarries and discovery of the postcranial remains attributable to the Nemegtosaurus holotype provides the first opportunity to directly compare these two taxa. Seven additional sites at the Nemegt locality preserve sauropod remains (including vertebrae, humeri, femora, pelvic elements, pedal phalanges, and unguals), and more than 20 sauropod footprint sites have been mapped. None of this material suggests that there is more than one sauropod taxon present in the Nemegt Formation. All localities occur within a discrete stratigraphic interval encompassing the uppermost Baruungoyot (footprints), Baruungoyot-Nemegt interfingering interval (Nemegtosaurus type), and lowermost Nemegt formations. Stratigraphic comparisons indicate the Opisthocoelicaudia locality at Altan Uul IV is within the lower beds of the Nemegt Formation. As sauropod remains are now documented for a total of 34 sites in the Nemegt Formation, a more refined stratigraphic framework may shed new light on the taxonomic inclusiveness of the sample.

Introduction

R. Gradziński discovered what became the holotype of the sauropod dinosaur Opisthocoelicaudia skarzynskii Borsuk-Bialynicka, 1977 at Altan Uul IV in early June 1965. The huge excavation, which was undertaken between June 25th and July 7th, 1965, was complicated because it had been found on an elevated shelf of rock surrounded by deep canyons and even higher badlands (Kielan-Jaworowska and Dovchin 1968). The closest the team could get vehicles to the site was more than half a kilometer, which meant that equipment, supplies, tools and fossils had to be carried or dragged by expedition members. Twelve tonnes of bone with the adhering hard sandstone were dragged on stone boats (made from empty fuel drums) to ‘the Café’, where they could be crated and loaded on trucks.

While that specimen was being excavated, a side trip to Nemegt led to the discovery of another sauropod. Kielan-Jaworowska (1969, p. 115) recounted in her expedition narrative that Dovchin, Gradziński, Kuczyński, Maryańska and a driver left on June 15th for a two-day fieldtrip to Nemegt. There, Kuczyński found a sauropod skull encased in the wall of one of the Nemegt canyons. They did not have the tools to remove more than a metre of dense sandstone above the skull. However, Walknowski drove another truck past the site a few days later en route to Gurvan Tes. He took along Kuczyński—the discoverer of the skull—and Skarżyński so that they could excavate the skull. They returned three days later with the skull cushioned by wood shavings in a spacious wooden box.

The unpublished field notes of Gradziński provide a little more information. Apparently, the members of the initial team left Altan Uul for Nemegt on June 15th at midday, Kuczyński discovered the skull on the 16th at midday, and they returned to Altan Uul IV on the 17th. The skull became the holotype of Nemegtosaurus mongoliensis Nowinski, 1971.

Opisthocoelicaudia skarzynskii is a nearly complete postcranial skeleton lacking only the skull and neck, whereas Nemegtosaurus mongoliensis is a skull and lower jaws that lack a postcranial skeleton. Nemegtosaurus and Opisthocoelicaudia were initially assigned to different sauropod subfamilies with vastly different tooth shapes, Dicraeosaurinae and Camarasauridae respectively. However, since the 1990s (Gimenez, 1992, Salgado and Calvo, 1992, Salgado and Calvo, 1997, Wilson and Sereno, 1998), both genera have been recognized as members of Titanosauria. Their coincident spatiotemporal distribution and non-overlapping skeletal parts have led to the persistent suspicion that they belong to the same species. The only way to determine if they are synonymous is to find a specimen that has skeletal parts that overlap both of the holotypes.

Here we report on recent field explorations that relocated the Opisthocoelicaudia quarry in 2007 and the Nemegtosaurus quarry in 2016. The Nemegtosaurus locality has produced additional bones that are compatible in size with the holotype skull of Nemegtosaurus and likely pertain to the same individual, providing the first opportunity to evaluate the relationship between Nemegtosaurus and Opisthocoelicaudia. Thirty-two Nemegt sites (in addition to the holotype quarries, Table 1) preserve sauropod remains (including vertebrae, humeri, pelvic elements, femora, pedal phalanges, and unguals), and more than six sauropod footprint sites have been mapped (Stettner et al., 2017 this volume). These numbers do not include other sauropod material collected by the Soviet (Rozhdestvensky in Nowinski, 1971), Russian-Mongolian (Kurzanov and Bannikov, 1983) and Polish-Mongolian expeditions (Madzia and Borsuk-Bialynicka, 2014). All localities occur within a discrete stratigraphic interval encompassing the uppermost Baruungoyot (footprints), Baruungoyot-Nemegt interfingering interval (Nemegtosaurus type), and lowermost Nemegt formations (Eberth, 2017, this volume; Fanti et al., 2017, this volume). Stratigraphic comparisons indicate the Opisthocoelicaudia locality at Altan Uul IV is within the lower beds of the Nemegt Formation. Now that sauropod remains are documented for a total of 34 sites in the Nemegt Formation across the Nemegt Basin, a more refined stratigraphic framework may shed new light on the taxonomic inclusiveness of the sample.

MPC-D, Institute of Paleontology and Geology of the Mongolian Academy of Sciences, Ulaan Baatar; ZPal, Palaeozoological Institute, Polish Academy of Sciences, Warsaw.

Section snippets

Opisthocoelicaudia type locality

The location of the quarry for the sauropod skeleton that ultimately became the holotype of Opisthocoelicaudia skarzynskii is marked on the map of Altan Uul IV (Gradziński et al., 1969, their Fig. 4). The hand-drawn map was skillfully drawn through triangulation of cairns constructed at strategic points at Altan Uul IV. However, the small scale of the map made it impossible to relocate the quarry between 2006 and 2008. Fortunately, Gradziński was still alive at that time and provided additional

Systematic paleontology

DINOSAURIA Owen, 1842

SAUROPODA Marsh, 1878

MACRONARIA Wilson and Sereno, 1998

TITANOSAURIA Bonaparte and Coria, 1993

NEMEGTOSAURIDAE Upchurch, 1995

NEMEGTOSAURUS Nowinski, 1971

Description

Only the bones recovered during the 2016 field season will be described in this paper. Although more of the specimen will be recovered when funds become available, it is unlikely to be done until 2018. A caudal centrum, femur, tibia, fibula, astragalus, and pedal ungual were found at the site that produced the holotypic skull of Nemegtosaurus mongoliensis in 1965, and presumably belong to the same individual.

The centrum (Fig. 4, Fig. 5, Fig. 6) seems to be from either a sacral or proximal

Discussion

Although they are less common than those of theropods and hadrosaurs, sauropod remains such as isolated teeth, postcranial elements and footprints are now known from multiple localities in the Nemegt Formation, including Altan Uul II, III and IV, Hermiin Tsav II, Nemegt, Tsagaan Khushuu, and Ulaan Khushuu (Table 1).

At the time of writing, eight sites within the Nemegt locality preserve sauropod remains (one in the Western Sayr, two in the Central Sayr, five in the Northern Sayr), and more than

Conclusions

Relocation of the quarries of the holotypes of Nemegtosaurus mongoliensis and Opisthocoelicaudia skarzynskii has led to the discovery of additional bones of Nemegtosaurus that allow for the first time direct morphological comparison between the two species. Currently, these include one caudal centrum, a femur, the distal ends of the tibia and fibula (both of which are still in situ in the field), an astragalus, and a pedal ungual. Each of these bones is very similar to corresponding elements in

Acknowledgments

Relocation of the Polish-Mongolian Paleontological Expedition quarries has only been possible because of the maps that were produced by R. Gradziński, T. Jerzykiewicz, and various people who assisted them. The Opisthocoelicaudia quarry was relocated in 2009 after R. Gradziński provided detailed notes, photographs and sketches to PJC. The Nemegtosaurus holotype quarry was not rediscovered until after Gradziński had passed away, but finding a means to confirm its discovery would not have been

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