Megalosauripus transjuranicus ichnosp. nov. A new Late Jurassic theropod ichnotaxon from NW Switzerland and implications for tridactyl dinosaur ichnology and ichnotaxomy.

Novella L Razzolini,Géraldine Paratte,Daniel Marty,Matteo Belvedere,Marielle Cattin,Christian A Meyer,Christel Lovis,Anthony Fiorillo

doi:10.1371/journal.pone.0180289

Abstract

A new ichnospecies of a large theropod dinosaur, Megalosauripus transjuranicus, is described from the Reuchenette Formation (Early–Late Kimmeridgian, Late Jurassic) of NW Switzerland. It is based on very well-preserved and morphologically-distinct tracks (impressions) and several trackways, including different preservational types from different tracksites and horizons. All trackways were excavated along federal Highway A16 near Courtedoux (Canton Jura) and systematically documented in the field including orthophotos and laserscans. The best-preserved tracks were recovered and additional tracks were casted. Megalosauripus transjuranicus is characterized by tridactyl tracks with clear claw and digital pad impressions, and notably an exceptionally large and round first phalangeal pad on the fourth digit (PIV1) that is connected to digit IV and forms the round heel area. Due to this combination of features, M. transjuranicus clearly is of theropod (and not ornithopod) origin. M. transjuranicus is compared to other Megalosauripus tracks and similar ichnotaxa and other unassigned tracks from the Early Jurassic to Early Cretaceous. It is clearly different from other ichnogenera assigned to large theropods such as Eubrontes–Grallator from the Late Triassic and Early Jurassic or Megalosauripus–Megalosauropus–Bueckeburgichnus and Therangospodus tracks from the Late Jurassic and Early Cretaceous. A second tridactyl morphotype (called Morphotype II) is different from Megalosauripus transjuranicus in being subsymmetric, longer than wide (sometimes almost as wide as long), with blunt toe impressions and no evidence for discrete phalangeal pad and claw marks. Some Morphotype II tracks are found in trackways that are assigned to M. transjuranicus, to M.? transjuranicus or M. cf. transjuranicus indicating that some Morphotype II tracks are intra-trackway preservational variants of a morphological continuum of Megalosauripus transjuranicus. On the other hand, several up to 40 steps long trackways very consistently present Morphotype II features (notably blunt digits) and do not exhibit any of the features that are typical for Megalosauripus (notably phalangeal pads). Therefore, it is not very likely that these tracks are preservational variants of Megalosauripus transjuranicus or Megalosauripus isp. These trackways are interpreted to have been left by an ornithopod dinosaur. The high frequency of large theropod tracks in tidal-flat deposits of the Jura carbonate platform, associated on single ichnoassemblages with minute to medium-sized tridactyl and tiny to large sauropod tracks has important implications for the dinosaur community and for paleoenvironmental and paleogeographical reconstructions. As with most other known occurrences of Megalosauripus tracks, M. transjuranicus is found in coastal settings, which may reflect the preference of their theropod trackmakers for expanded carbonate flats where food was abundant.

Highlights

Megalosauripus can be considered as one of the most widespread Late Jurassic ichnotaxa made by large theropods in Europe, America, and Asia
Megalosauripus is known from Late Jurassic to Early Cretaceous deposits, it has been described from the Middle Jurassic of Asia, North America and Europe [3,7,8,9,10,11,12,13]
The purpose of this paper is to describe various new, and well-preserved large tridactyl tracks from the Late Jurassic of Highway A16 (NW Switzerland) from a morphological point of view

Summary

Introduction

Megalosauripus can be considered as one of the most widespread Late Jurassic ichnotaxa made by large theropods in Europe, America, and Asia. Its correct assignment and validity has been highly debated in the last twenty years [1,2,3,4,5,6] This is especially because this ichnotaxon represents the typical shape of a large theropod print (tridactyl, longer than wide, narrow), which is morphologically conservative and difficult to characterize and distinguish. The ichnotaxonomical entanglement started in 1955, when [14] coined the name Megalosauripus referring to an illustration of a track from the German collection of Ballerstedt [15], and reproduced in [16], assumed to have been left by a megalosaur dinosaur Because this track was named after the purported trackmaker, and no proper ichnotaxonomical description was provided, this ichnogenus was declared a nomen nudum in [1] and formalized as Megalosauripus ichnogen. We follow the use of Megalosauripus sensu [3] because of the morphological affinity of the studied material with the ichnogenus definition and the emended description of [6]

Objectives

Methods

Findings

Conclusion