Volcanic preservation is considered very unusual in Palaeontology. Ashfalls sometimes preserve tissues, and more often traces such as footprints or burrows, but contact with hotter volcanic material is extremely damaging to most organic matter, and therefore rarely seen. Where it does occur it generally preserves only the toughest tissues (wood, bone etc.), and then only if they were rapidly engulfed by volcanic material that excluded all oxygen, thereby preventing combustion.
In a paper published in the journal PLoS One on 21 November 2012, a team of palaeontologists and volcanologists led by Pierre-Olivier Antoine of the Institut des Sciences de l’Évolution at Université Montpellier describe a 9.2 million-year-old (Late Miocene) Rhiniceros skull from an ignimbrite (preserved ash-flow) in the Çardak Caldera in Cappadocia in central Anatolia.
The skull is assigned to the species Ceratotherium neumayri, a two-horned Rhino well known from the Late Miocene of the eastern Mediteranean, and the ignimbrite is thought to have been formed by an ancient pyroclastic flow (avalanche of hot ash and gas).
Karacaşar exposure, Cappadocia, Central Anatolia, Turkey. Detail of the exposure (a), showing the cerebellar section of the Ceratotherium neumayri cranium cropping out in the bank of the stream (b), and the right angulus mandibulare as later appearing during the extraction process (c). Note the presence of bone fragments to the left of the skull (i.e. South to it) and of centimetric whitish pumice clasts both within and around the cerebellar area (b, c). Antoine et al. (2012).
Both the foreparts and back of the skull are missing, however the rear of the skull appears to have been eroded away by the stream wearing through the rock that led to its exposure and discovery, while the front of the skull show signs of abrasion before or during the burial process, with fine ash covering the worn surfaces. The skull shows an unusual mode of preservation, with a rough surface, broken and disintegrated tooth roots and brittle dentine.
Heat and fire damage are unusual in palaeontology (the best way for anything to get fossilized is for it to be buried rapidly underwater in an oxygen-poor environment), but the effects of fire on tissues have been well studied by archaeologists and forensic scientists, providing a body of data from which Antoine et al. were able to make several deductions.
Ceratotherium neumayri, Karacaşar (Anatolia, Turkey), late Miocene (~9.2 million years old). Articulated cranium and mandibl. (a) Left lateral view, with upper/lower cheek teeth angle (ca. 26˚) and tentative reconstruction of the lacking parts (maxillae, nasals, parietals, and occipital bone). (b) Upper cheek tooth series, with left P2-M3, in occlusal view. (c) Lower cheek tooth series, with left p2-m3, in labial-occlusal view. The corrugated aspect of the bony surface (3a, 3c) is interpreted as resulting to a long exposure to warm volcaniclastics. Scale bar is 50 mm. Antoine et al. (2012).
The skull has a pinkish colour to it, which Antoine et al. attribute to the mineralogy of the surrounding ash matrix. The rough surface of the skull, as well as the numerous cracks that cover it and the disintegration of the roots of the teeth, is attributed to rapid dehydration of the bone, due to exposure to temperatures in excess of 400˚C. The surfaces of the teeth, when examined under a microscope, show widely spaced linear cracks, indicative of direct exposure to material at temperatures in excess of 450˚C. The mouth of the Rhino is preserved in a wide-open position, which is highly unusual, and which is considered to be a result of soft tissue dehydration and protein denaturing, again signs of exposure to high temperatures
Taphonomical processes involved in the preservation of the Karacaşar Rhino. (a) General view of the cranium and mandible. (b) Detail of the dorsal area of the cranium, with a broken rhino rib, trapped ‘upstream’ beside it. (c) Detail of baked dentine (brittle occlusal surface) and roots (hollow) of right P4-M2. (d) Labial detail of left p3-m1 showing deteriorated roots and intact enamel; note that p4 and m1 show mild enamel hypoplasia. Scale bar is 20 mm. Antoine et al. (2012).
Antoine et al. consider that the Rhino almost certainly died as a result of being hit by a pyroclastic flow, and that the damage seen to the bone tissues is the result of baking within the extreme heat of the ash-flow. They believe the body to have been moved some way by the pyroclastic flow after death, the head becoming detached in the process.
Heating effects on the Karacaşar rhino skull as revealed by light microphotographs of hard tissues. a. Detail of the crown of the right p3, with large cracks running through the dentine and oblique cracks affecting the enamel, separation of the enamel-dentine junction (secondarily re-crystallized) and dense crack network affecting both the enamel and dentine, indicating direct exposure to a ≥400–450°C heat (scale bar 1.0 mm). (b) Control rhino tooth (unheated), without structural change (scale bar 1.0 mm). c. Detail of the root of the right p3, with cracks passing across the cementum and dentine, and longitudinal cracks within the dentine, pointing to a ≥450°C heating exposure (scale bar, 1.0 mm). d. Thermal modification of cortical compact bone (right nasal bone): presence of linear and polygonal cracking between osteons (scale bar 0.2 mm). e. Same thin section as in (d), but magnified twice (scale bar 0.1 mm). Osteons, osteocyte lacunae, and haversian canals are preserved. Lamellae and canaliculi have vanished, thus pointing to short and controlled exposures at ≥300–400˚C. Antoine et al. (2012).
Antoine et al. were (surprisingly) able to find two other cases of volcanically preserved Rhinos. The first being the Blue Lake Rhino of Grand Coulee in Washington State, USA, where a dead Rhino floating upside down in a lake was moulded within basalts emerging as pillow lavas beneath the water. The second is from an 11.8 million-year-old ashfall caused by an eruption in the Bruneau-Jarbridge Caldera in southwest Idaho, where a large number of animals, including several Rhinos, were killed and then buried by a volcanic ashfall over a period of about four weeks.
Location map of the Karacaşar Rhinocerotine skull, late Miocene of Cappadocia, Central Anatolia, Turkey. The skull (‘‘rhino site’’) was unearthed from the 9.260.1 million-year-old Kavak-4 subunit of the upper Miocene Kavak ignimbrite, 3 km east of Karacaşar. The pyroclastic matrix of the skull probably sourced from the Çardak Caldera, 30 km to the south. Antoine et al. (2012).
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