Synchrotron X-rays Reveal Identity of 1.5 million-year-old Tuscan Big Cat

29-05-2018

The identity of a mysterious fossil felid found in central Italy has been revealed thanks to synchrotron techniques. Scientists used X-ray tomography to virtually extract the fossil from its rock encasing and describe decisive anatomical details for the first time. Previously thought to be an extinct Eurasian jaguar, this new study concluded by identifying the felid as Acinonyx pardinensis, one of the most intriguing extinct carnivores of the Old World Plio-Pleistocene. The study is published in Scientific Reports.

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The team of physicists and palaeontologists from the University of Perugia, the University of Verona and the University of Rome Sapienza, in collaboration with the European Synchrotron, ESRF, scanned the partial skull of the specimen, still embedded in the rock. The analysis of images and 3D models obtained revealed a mosaic of cheetah-like teeth and Panthera-like features leading to a reconsideration of the ecological role of this species.

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Right lateral view of specimen ArgBsc1 of Acinonyx pardinensis from Monte Argentario. Credit: Dawid A. Iurino

The skull was discovered on Monte Argentario in Tuscany at the beginning of the 20th century and belongs to the Villafranchian Land Mammal Age. This period is significant because the earliest hominids that clearly evolved into modern man appeared within it. Many animals and their extinct ancestors also evolved during this period including the Etruscan bear, sabre-toothed cats, mammoths and the earliest fallow deer. Until today, and because of the fossil’s imprisonment in the rock, the observation of the anatomical details needed for taxonomic determination was impossible without damaging the fossil, hence the debate over its identity. Although regularly used for hominid or dinosaur fossils, this is the first time synchrotron microtomography has been used to scan a carnivoran mammal fossil. X-ray based tomographic methods developed at the ESRF allowed the scientists to virtually extract the fossil from its host matrix, a reddish rock block with a similar density to the fossil. The combination of hard X-rays, high beam intensity and partial coherence provided huge amounts of information on the external and internal morphology of the specimen down to the micrometre scale.  “Imaging such large fossils has become a typical characteristic of the ESRF. The X-rays generated here allow us to solve problems and get information that would be very difficult to obtain elsewhere,” says Vincent Fernandez, palaeontologist, co-author from ESRF’s ID19 beamline. 

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Dawid Iurino with the Acinonyx pardinensis skull from Monte Argentario, on the set-up of ESRF ID17 beamline. Credit: Marco Cherin

High-resolution 3D images obtained at the ESRF’s ID17 beamline revealed previously hidden anatomical features, including sutures between the bones of the palate and the complete morphology of the teeth. The latter are just some of the elements that made it possible to refer the skull to the species Acinonyx pardinensis, the Plio-Pleistocene giant cheetah.

Palaeontologists consider Acinonyx pardinensis as one of the most intriguing carnivore mammals of the recent geological past in the Old World. Some of its anatomical characters, for example the short snout, pointed premolar and molar teeth, elongated body with long limbs and long tail, resemble the living cheetah (Acinonyx jubatus). Other features, however, like the large body size weighing some 80-90 kgs, the jaw muscle system and shape of the braincase resemble pantherine cats, such as the jaguar or leopard.

The co-occurrence of Acinonyx-like and Panthera-like characteristics has allowed scientists to hypothesise that A. pardinensis had a hunting strategy and killing behaviour different to those of the living cheetah, and more in line with those of an adult jaguar.

 “The study on the Monte Argentario felid was the starting point for a review of the entire Old World sample of A. pardinensis. We discovered that, regardless of the age of the different sites (the species inhabited Eurasia and North Africa for more than 2 million years), we can recognize a great body size variation between individuals”, says Marco Cherin, lead author of the publication. “This can be explained if we assume that A. pardinensis, like all extant large felids, was characterised by sexual dimorphism, meaning that males of the species are larger and stouter than the females.”

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3D models of Acinonyx pardinensis skull from Monte Argentario. Left to right: damaged and fragmented skull in rostral view with embedding sediment, without sediment and reconstructed skill with cloned and mirrored portions highlighted in different colour.  Credit: Dawid A. Iurino

Slower but stronger

Among the big cats today the cheetah stands out for its extraordinary sprinter skills that allow it to reach 100 km/h in a few seconds. The rapid acceleration allows the predator to suddenly approach its prey, mostly gazelles and antelopes, which are then landed and suffocated by a bite to the throat. To achieve this performance, the cheetah's skeleton has evolved over time, becoming thinner and smaller in size, with only partially retractile claws and with a small rounded skull with large nasal cavities. These, in fact, favour a greater flow of air during the pursuit, but the price of expanded nasal cavities is a reduction in the size of the canines, greatly reduced in cheetahs compared to other large cats. Overall, this series of adaptations is expressed in a style of hunting that is practically unique among today's carnivores.

The characteristics observed on the fossils of A. pardinensis, however, tell of an unusual skeletal structure for a cheetah. As revealed by the study, the estimated average size of about 80 kg is similar to that of an adult jaguar, as well as the tell-tale robust bones and massive teeth. A cheetah of this size could not achieve the same high performance in a pursuit as its current descendants.  The anatomy of A. pardinensis is in some respects reminiscent of the current, slower and sturdier jaguar, with ambush hunting behaviour involving short pursuits and powerful bites to the nose or throat. The Plio-Pleistocene giant cheetah was therefore a felid capable of bringing down large herbivores in the same way as today's lions or jaguars. These characteristics make it one of the top predators in the trophic network of the Plio-Pleistocene ecosystems, next to sabre-toothed cats and giant hyenas.

References

Cherin M., et al., Synchrotron radiation reveals the identity of the large felid from Monte Argentario (Early Pleistocene, Italy). Scientific Reports, 29 May 2018. Doi:10.1038/s41598-018-26698-6.

Text by Kirstin Colvin

Top image: Reconstruction of the head of Acinonyx pardinensis, based on the complete skull from Pantalla (Italy) and updated with the information on the craniodental anatomy of A. pardinensis achieved from the Monte Argentario specimen. Artwork by Dawid A. Iurino.