ESRF Highlights 2020

149HIGHLIGHTS 2020

Marginal dentition and multiple dermal jawbones as the ancestral condition of jawed vertebrates, V. Vaškaninová (a,b), D. Chen (a), P. Tafforeau (c), Z. Johanson (d), B. Ekrt (e), H. Blom (a) and P.E. Ahlberg (a),

Science 369, 211-216 (2020); https://doi.org/10.1126/science.aaz9431. (a) Uppsala University, Uppsala (Sweden) (b) Charles University, Prague (Czech Republic)

[1] M. Zhu et al., Science 354, 334-336 (2016). [2] D. Chen et al., Nature 539, 237-241 (2016).

UNRAVELLING THE FINGERPRINTS OF DEEP CRUSTAL EARTHQUAKES

When continents collide, the Earth s crust experiences stress levels that cause dynamic rupturing and seismic slip during earthquakes. Lower crustal earthquakes release large amounts of energy in small volumes of rocks and produce characteristic damage patterns in and around the slip surface. X-ray microtomography provides key information about the dynamics of the processes involved.

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land animals, acanthothoracids only added new teeth on the inside; the oldest teeth were located right at the jaw margin. The tooth-bearing bones also carry small, non-biting dentine elements of the skin on their outer surfaces, a character shared with primitive bony fish [2] but not with arthrodires. This is an important difference because it shows that acanthothoracid jaw bones were located right at the edge of the mouth, whereas arthrodire jaw bones lay further in. Uniquely, one acanthothoracid (Kosoraspis) shows a gradual shape transition from these dentine elements to the neighbouring true teeth (Figure 131), while another (Radotina) has true teeth almost identical to its skin dentine elements in shape. This may be evidence that the true teeth had only recently evolved from dentine elements on the skin.

These findings change our whole understanding of the origin of teeth. Even though the acanthothoracid dentitions perfectly match the model for a common ancestral dentition for cartilaginous and bony vertebrates, it was

Fig. 131: Virtual render of the teeth and jawbones (yellow) of the acanthothoracid Kosoraspis articulated with the ethmoid (violet).

not expected to be discovered among the most primitive of all jawed vertebrates. In addition, acanthothoracid jawbones resemble those of bony fish and seem to be directly ancestral to our own.

When continents collide, a mountain range forms along the collision zone in an area referred to as an orogenic belt. The evolution of orogenic belts depends on the physical properties of the different layers within the lithosphere. Since the early 1980s, the most popular model of lithospheric strength included a mechanically weak lower crust. Recent research challenges this model [1], claiming that the lower crust is strong. A key observation is that lower crustal

rocks locally display faults (fractures with significant displacement) filled with a layer of pseudotachylyte, a rock type believed to represent frozen frictional melt formed during an earthquake. Synchrotron X-ray computed microtomography (µ-CT) and electron back- scattered diffraction (EBSD) allows accurate determination of the structure of such faults and detailed characterisation of the mechanical damage associated with it.