An old bat from Wyoming

The Lower Eocene Green River Formation of Wyoming is dominated by fine-grained lake sediments, mainly made of laminated limy mudstones. Many layers constitute superb lagerstätten teeming with remains of delicate organisms. As well as much else, The Green River Formation is noted for its early bats, which suddenly appear in the fossil record with all the prerequisites for flight. The cover of the 14 February 2008 issue of Nature depicts a perfect specimen showing the four elongated ‘fingers’ that supported its wing membrane, and a long tail, which few modern bats have, except in atrophied form to support the rear part of the wing. In many respects it has a transitional structure between non-flying mammals and later bats, but would definitely have been a good flyer or rather flutterer-glider.

Not only is the fossil spectacularly well-preserved, detail of its head morphology helps resolve the issue of whether echolocation preceded flight (Simmons, .B. et al. 2008. Primitive Early Eocene at from Wyoming and the evolution of flight and echolocation. Nature, v. 451, p. 818-821). Other, slightly later fossil bats from the Green River Formation probably did echolocate, as evidenced by their stomach contents, and enlarged larynx and cochlea for transmitting and receiving the now typical high pitched squeaks of many bats. Onychonychteris doesn’t have such characteristics, so it seems as if echolocation did not evolve before flight, thereby resolving one of Darwin’s vexations about the universality of natural selection. Prior to the discovery by Simmons et al. many bat-oriented evolutionists speculated that echolocation evolved among small arboreal mammals so that they could detect passing insects. A habit of leaping to grab the prey in turn selected for an ability to glide from a strategic perch, for quite obvious reasons. Success further encouraged the evolution of powered flight. Yet no other living mammals have echolocation, probably because it is a highly energy-intensive habit. However, the muscles used by a flying mammal serve also to make squeaking a ‘cost-free’ bonus. So, the findings in Onychonychteris seem to resolve the matter nicely.

See also: Speakman, J. 2008. A first for bats. Nature, v. 451, p. 774-775.

Life perked up by repeated impacts

Following the blazes of publicity since the early 1980s about the demise of the dinosaurs at the K/T boundary it is easy to regard objects the size of mountains that fall out of the sky as bad news for life. That is despite the fact that, bar the Chicxulub impact structure that exactly matches the timing of the end-Cretaceous mass extinction, no other significant and rapid drop in the diversity of life has been found to be associated with an extraterrestrial impact. Whatever their cause, mass extinction events sometimes seem to be followed by bursts in biodiversity, presumably as the survivors eventually find lots of new opportunities and diversity to occupy them. One exception is the end-Ordovician mass extinction that was also preceded by a tripling in the number of families, which the extinction rudely interrupted. This has often been seen as a somewhat delayed exploitation of all the advantages and competitive opportunities conferred by the appearance of hard parts at the start of the Cambrian. But remarkable finds in the limestone-rich Ordovician of Scandinavia suggest an unexpected connection with meteorite bombardment (Schmitz, B. and 8 others 2008. Asteroid breakup linked to the Great Ordovician Biodiversification Event. Nature Geoscience, v. 1, p. 49-53).

The most usual measure of diversity used by stratigraphic palaeontologists is the number of families at a particular time, and the overall tripling in the Middle to Upper Ordovician is notable. However, if specimens of individual groups, such as brachiopods, are collected from the Scandinavian limestones on a bed by bed basis, increased diversity at the species level is even more dramatic. There are sudden doublings or triplings over periods of what can be no more than a few hundreds of ka, especially around 470 Ma ago. In the 1960s potassium-argon dating of chondritic meteorite collections revealed a cluster of reheating ages between 500 and 450 Ma (Upper Cambrian to Upper Ordovician); about 20% of all meteorites fall into this age-cluster, and most show evidence of having been shocked as well as heated up. This seems to signify a major collision or series of collisions in the Asteroid Belt around the early Palaeozoic. More reliable and precise 40Ar-39Ar dating narrows this event to a period between 463 and 477 Ma in the Middle Ordovician. In 2001, Birger Schmitz of the University of Lund reported, with others, more than 50 sizeable chondritic meteorites in the Middle Ordovician limestones of Sweden. Schmitz and his Damnish, US and Chinese colleagues in the new paper give plots of brachiopod species and also the abundance of chromite grains of meteoritic origin in Middle Ordovician limestones from Sweden and China. Two sharp jumps in brachiopod species numbers are  preceded and accompanied by ‘spikes’ in the number of extraterrestrial chromite grains, so the link seems to be real. Yet what can have produced such a counter-intuitive result? One possibility is that the undoubted disturbance may have killed off species of one group, maybe trilobites, so that the resources used by them became available to more sturdy groups, whose speciation filled the newly available niches. Such a scenario would make sense, as mobile predators/scavengers (e.g. trilobites) may have been less able to survive disruption, thereby favouring the rise of less metabolically energetic filter feeders (e.g. brachiopods).


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