Monday, January 11, 2016

[Paleontology • 2016] Machimosaurus rex • The Largest Thalattosuchian (Crocodylomorpha) supports Teleosaurid Survival Across the Jurassic-Cretaceous Boundary


Machimosaurus rex Fanti, Miyashita, Cantelli, Mnasri, Dridi, Contessi & Cau, 2016
Life reconstruction: Davide Bonadonna theropoda.blogspot.com
 DOI: 
10.1016/j.cretres.2015.11.011 

Fig. 8. Comparison among skulls of Machimosaurus.
 (A) holotype of M. buffetauti, (B) neotype of M. mosae, (C) estimated size of the ‘Leira specimen’ of M. hugii,
 (D) holotype of Machimosaurus rex. Dashed areas in (A) and (B) indicate size of largest known individuals of those species.
(E) Reconstruction of 
Machimosaurus rex body based on preserved elements.
Figures (A)–(C) modified from Young et al. (2014b).  DOI: 10.1016/j.cretres.2015.11.011  

Highlights
Machimosaurus rex is a new teleosaurid crocodylomorph from Tunisia.
• It is the largest known thalattosuchian, up to 10 m in length.
M. rex, the first Cretaceous teleosauroid found, was the last-surviving of its group.

Abstract
A new teleosaurid from the Lower Cretaceous of Tataouine (Tunisia), Machimosaurus rex sp. nov., definitively falsifies that these crocodylomorphs faced extinction at the end of the Jurassic. Phylogenetic analysis supports its placement closer to M. hugii and M. mosae than M. buffetauti. With the skull length up to 160 cm and an estimated body length of 10 m, M. rex results the largest known thalattosuchian, and the largest known crocodylomorph at its time. This giant thallatosuchian probably was an ambush predator in the lagoonal environments that characterized the Tethyan margin of Africa during the earliest Cretaceous. Whether the Jurassic-Cretaceous mass extinction was real or artefact is debated. The discovery of M. rex supports that the end-Jurassic crisis affected primarily Laurasian biota and its purported magnitude is most likely biased by the incomplete Gondwanan fossil record. The faunal turnovers during the J-K transition are likely interpreted as local extinction events, triggered by regional ecological factors, and survival of widely-distributed and eurytypic forms by means of habitat tracking.

Keywords: Lower Cretaceous; Machimosaurus; Teleosauridae; Thalattosuchia; Tunisia


Systematic paleontology

Crocodylomorpha Hay, 1930
Thalattosuchia Fraas, 1901

Teleosauridae Saint-Hilaire, 1831
Machimosaurus von Meyer, 1837

Machimosaurus rex sp. nov.
(ZooBank code: LSID urn:lsid:zoobank.org:act:1A11E9B9-0B1C-4557-92B7-165168658C17)

Etymology. The species name rex, Latin for “king”, refers to its majestic size among known Machimosaurus and all thalattosuchians.

Holotype. ONM NG NG 1–25, 80, 81, and 83–87
( Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6 and Fig. 7D; Table 1).

Fig. 4. Machimosaurus rex type skull,
(Ain situ photograph showing dorsally exposed preserved bones, (B) prepared ventral surface.
Abbreviations: fr, frontal; lj, left jugal; la, lacrimal; ld, left dentary; lmx, left maxilla; lna, left nasal; lpd, left postdentary elements; lposq, left postorbitalsquamosal bar; os, osteoderm; pa, palatal element; rd, right dentary; rmx, right maxilla; rna, right nasal; rpd, right postdentary elements; rposq, right postorbital-squamosal bar; stfo, floor of supratemporal fossa; tp, turtle plastron element. Scale bar = 50 cm.

Locality and horizon. Touil el Mhahir, Tataouine Governorate, Tunisia; Douiret Sand Member, Douiret Formation, Hauterivian, Lower Cretaceous.

Fig. 1. (A) Geographic location and type locality of Machimosaurus rex. (B) Simplified geological map of the Tataouine basin of southern Tunisia showing the Touil el Mhahir locality.

Diagnosis. Teleosaurid differing from other species by unique combination of: adult basicranial length >155 cm ( Fig. 5); rostrum ornamented by densely arranged, parallel longitudinal ridges; orbit elliptical; interorbital space narrow (one fifth length of skull posterior to orbit); anteromedial margin of supratemporal fossae round; frontal not extended anteriorly to orbit and with reduced orbital margin; relatively large maxillary alveoli; anterior dorsal neural spine height less than centrum height; dorsal osteoderms with tightly packed pits that are round centrally and ellipsoid peripherally.


Fig. 8. Comparison among skulls of Machimosaurus
(
A) holotype of M. buffetauti, (B) neotype of M. mosae, (C) estimated size of the ‘Leira specimen’ of M. hugii(D) holotype of Machimosaurus rex. Dashed areas in (A) and (B) indicate size of largest known individuals of those species. (E) Reconstruction of Machimosaurus rex body based on preserved elements.
Figures (A)–(C) modified from Young et al. (2014b).  DOI: 10.1016/j.cretres.2015.11.011 


Machimosaurus rex Fanti, Miyashita, Cantelli, Mnasri, Dridi, Contessi & Cau, 2016
Artwork by Davide Bonadonna theropoda.blogspot.com
DOI: 
10.1016/j.cretres.2015.11.011

Conclusion: 
Machimosaurus rex sp. nov. is based on the articulated skeleton of a giant crocodylomorph from the Hauterivian of Tunisia. This taxon represents the first indisputable Cretaceous teleosauroid, and the first member of this clade from Africa based on well preserved remains. With a basicranial length approaching 160 cm (and a partial skeleton indicating a total body length around 10 m), M. rex is the largest known thalattosuchian. Both paleoecological data and morphological features suggest that this species was an ambush generalist predator with an ecology comparable to extant semi-aquatic crocodilians. The discovery of M. rex falsifies a global mass extinction event at the J-K transition (i.e., teleosauroid extinction), thereby highlighting the problem of sampling bias in the reconstruction of large-scale patterns in the geological record. The new Tunisian teleosaurid points to a conservative interpretation of faunal turnovers during the J-K transition: local extinction events triggered by regional ecological factors and survival of widely-distributed and eurytypic forms by means of habitat tracking.


Federico Fanti, Tetsuto Miyashita, Luigi Cantelli, Fawsi Mnasri, Jihed Dridi, Michela Contessi and Andrea Cau. 2016. The Largest Thalattosuchian (Crocodylomorpha) supports Teleosaurid Survival Across the Jurassic-Cretaceous Boundary. Cretaceous Research. In Press. DOI: 10.1016/j.cretres.2015.11.011 

1 comment:

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