Ordovician fish from the Arabian Peninsula

International audienceOver the past three decades Ordovician pteraspidomorphs (armoured jawless fish) have been recorded from the fringes of the Gondwana palaeocontinent, in particular Australia and South America. These occurrences are dominated by arandaspid agnathans, the oldest known group of vertebrates with extensive biomineralisation of the dermoskeleton. Here we describe specimens of arandaspid agnathans, referable to the genus Sacabambaspis Gagnier, Blieck and Rodrigo, from the Ordovician of Oman, which represent the earliest record of pteraspidomorphs from the Arabian margin of Gondwana. These are among the oldest arandaspids known, and greatly extend the palaeogeographical distribution of the clade around the periGondwanan margin. Their occurrence within a very narrow, nearshore ecological niche suggests that similar Middle Ordovician palaeoenvironmental settings should be targeted for further sampling

The tail of the Ordovician fish Sacabambaspis

The tail of the earliest known articulated fully skeletonized vertebrate, the arandaspid Sacabambaspis from the Ordovician of Bolivia, is redescribed on the basis of further preparation of the only specimen in which it is most extensively preserved. The first, but soon discarded, reconstruction, which assumed the presence of a long horizontal notochordal lobe separating equal sized dorsal and ventral fin webs, appears to have considerable merit. Although the ventral web is significantly smaller than the dorsal one, the presence of a very long notochordal lobe bearing a small terminal web is confirmed. The discrepancy in the size of the ventral and dorsal webs rather suggests that the tail was hypocercal, a condition that would better accord with the caudal morphology of the living agnathans and the other jawless stem gnathostomes

Newly formed craters on Mars located using seismic and acoustic wave data from InSight

Meteoroid impacts shape planetary surfaces by forming new craters and alter atmospheric composition. During atmospheric entry and impact on the ground, meteoroids excite transient acoustic and seismic waves. However, new crater formation and the associated impact-induced mechanical waves have yet to be observed jointly beyond Earth. Here we report observations of seismic and acoustic waves from the NASA InSight lander’s seismometer that we link to four meteoroid impact events on Mars observed in spacecraft imagery. We analysed arrival times and polarization of seismic and acoustic waves to estimate impact locations, which were subsequently confirmed by orbital imaging of the associated craters. Crater dimensions and estimates of meteoroid trajectories are consistent with waveform modelling of the recorded seismograms. With identified seismic sources, the seismic waves can be used to constrain the structure of the Martian interior, corroborating previous crustal structure models, and constrain scaling relationships between the distance and amplitude of impact-generated seismic waves on Mars, supporting a link between the seismic moment of impacts and the vertical impactor momentum. Our findings demonstrate the capability of planetary seismology to identify impact-generated seismic sources and constrain both impact processes and planetary interiors