Wednesday, 25 January 2017

Prothero's Guide to Prehistoric Mammals

Princeton University Press ISBN: 9780691156828
This is an admirable effort to teach extinct mammals by showing them in  context with extant species. I found it helpful to view fossil skulls and skeletons alongside photos of familiar animals. As an additional aid, most chapters have one or more phylogenetic trees culled from the recent literature. Of course, there are extinct orders with a tenuous connection to living ones; inevitably a long chapter deals with orders that do not fit any scheme (e.g. Dinocerata).
Skeleton of Eobasileus - from the Order Dinocerata,
which is difficult to place in the mammalian tree.
Galerie de Paléontologie et d'Anatomie Comparée, Paris.
Donald R. Prothero is an experienced author and has penned a well written and informative text. The illustrations by Mary Persis Williams reconstruct the body forms of prehistoric mammals, usually with several in the same figure and with a human silhouette to give a sense of scale. These drawings are quite restrained compared to those of Velizar Simeonovski in Horned Armadillos and Extinct Madagascar (the cover is an exception).
Skeleton of the huge hedgehog Deinogalerix koenigswaldi.
National Museum of Natural History, Leiden, the Netherlands
Photo by Peter Maas (CC BY-SA 3.0) 
The book is aimed at a broad readership as "Princeton Field Guide" might suggest. The publishers have encouraged depiction of large and spectacular mammals at the expense of smaller ones. There is poor coverage of rodents. On the other hand it is the large fossils that are on display in museums of natural history.
Reconstruction of the ground sloth Eremotherium.
Exhibit in the Fernbank Museum of Natural History,
Atlanta, Georgia. Photo by Daderot (CC0)
Indeed, a field guide to fossils might be useful on museum visits. Yet though there are many excellent photos of exhibits, the museums are not identified. The two pages of Illustration Credits are of little help. The credit line to the above Eremotherium figure is merely "Daderot/Wikimedia Commons."

Figures showing phylogenetic trees suffer from a similar deficiency. One from the rodent chapter is credited to E.T. Prothero. It is redrawn from a paper on the Laotian rock rat (Laonastes) by Huchon et al. (here). The reader is ill served by not being guided to the source papers.

Tuesday, 10 January 2017

Caecilian offspring feed on the skin or oviduct lining of the mother

A caecilian (Dermophis mexicanus)
Franco Andreone CC BY-SA 2.5
Caecilians are limbless amphibians with a burrowing lifestyle. There are oviparous and viviparous species. In the former there is an extended period of parental care. The hatchlings have sharp teeth which they use to feed on the outer layer of the parent's skin. Because skin feeding occurs in both African and South American species, it probably was evolved more than 100 million years ago (argued here).

Viviparous species take it one step further. The fetuses have teeth and feed on the hypertrophied lining of the oviduct (described here). This is just one of several forms of matrotrophy practiced by invertebrates (here) and vertebrates (here).

Thursday, 5 January 2017

Slow incubation of dinosaur eggs: why only birds survived

Cast of Mossospondylus eggs and embryo
Photo from Royal Ontario Museum by Daderot (CC)
Most people have heard that birds are dinosaurs. But that raises an obvious question: why did birds survive the end-Cretaceous mass extinction event that did for their non-avian kin? A paper just published in PNAS suggests this might be explained by different reproductive strategies.

Reconstruction of Protoceratops andrewsi
AntoninJury (Wikimedia Commons) CC BY-SA 4.0
In brief, the authors made CT scans of teeth from fossilized dinosaur embryos. They then counted the von Ebner lines, which reflect the incremental pattern of dentine formation. Applying some quite reasonable assumptions to the data, they estimated the incubation time of the dinosaur eggs as minimum 2.8 months for Protoceratops andrewsi and 5.8 months for Hypachrosaurus stebingeri. The incubation times of modern birds tend to be much shorter though the upper end of the range (11-85 days) overlaps with P. andrewsi.

It is suggested that the relatively long generation times of non-avian dinosaurs put them at a disadvantage in competing with birds, reptiles and mammals during the Cretaceous-Palaeogene transition.