Friday, 10 March 2017

Physiology of dinosaurs

Barosauras lentus rearing: mounted skeleton at AMNH
Photo by Greg CC BY 2.0 
In a previous post I discussed the increase in brain metabolic rate during hominin evolution. Roger Seymour, who co-authored that study, has used similar insights to analyse the cardiovascular physiology of sauropod dinosaurs (open access). 

One of his conclusions is that if Barosaurus reared its head (as in the exhibit above) its mean arterial blood pressure would have been 700 mmHg and the left ventricle of the heart would have weighed a metric ton. The heart would have filled the chest and its thick wall been so stiff as to consume a huge amount of energy.
Diplodocus carnegii in the hall of the Natural History Museum in
South Kensington (now removed). Photo by Drow male CC BY-SA 4.0
Seymour goes so far to suggest that long-necked sauropods like Diplodocus were aquatic, floating on the surface of the water and lowering their heads to browse on vegetation. Abdominal air sacs would have helped it to float.

Sauropods are generally regarded as terrestial because of skeletal features, but perhaps they retained these because they needed to go on land to lay their eggs!
Dinocephalosaurus a live-bearing archosauromorph
From Liu et al. 2017
Large reptiles that became fully aquatic were viviparous as shown in a recent paper by Liu et al. (open access). This particular example was an archosauromorph and thus in the lineage that gave rise to dinosaurs (including birds) and crocodilians.

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.

Tuesday, 13 December 2016

Carnegie Collection of human embryos

Carnegie embryo 8171. Early lacunar stage (Stage 5b)
Courtesy of Dr. Allen C. Enders
An important source for human embryology, including implantation and formation of the placenta, is the Carnegie Collection now housed at the Human Developmental Anatomy Center in Washington D.C. The core of this collection is the carefully dated series of embryos first described by Hertig, Rock and Adams (here).

The Virtual Human Embryo is an online ressource based on the serially sectioned embryos in this collection and includes 3D reconstructions. It covers all 23 Carnegie stages in the first 8 weeks of embryonic development and cannot be too highly recommended.

Carnegie Embryo 7801. Showing extraembryonic coelom (eec)
and secondary yolk sac (sys) (Stage 6)
Courtesy of Dr.Allen C. Enders
Now a group in Amsterdam has used the Carnegie Collection to develop an additional annotated digital atlas of human development (described here). They also utilized material from the Boyd Collection at the Centre for Trophoblast Research in Cambridge.

They make two claims. First that representations in textbooks have become increasingly schematic. This is demonstrably true. Second that the descriptions in standard texts are often based on extrapolation to humans from animal models. It is hard to assess if the latter truly is the case. For example Human Embryology by Hamilton, Boyd and Mossman (previous post) was based on the human embryos in the possession of the three authors. In Germany there was a strong tradition to cover the embryology of all vertebrates, concluding with the human, exemplified by Dietrich Starck's Embryologie.

In physiology, on the other hand, animal data often are presented as if they were human. One example concerns oxygen tensions in various parts of the fetal circulation. Pretty much every textbook of physiology has a large illustration of the fetal circulation with data obtained in sheep by Dawes, Mott and Widdicombe. The figure legends often fail to acknowledge the source or the species or both.

Wednesday, 26 October 2016

Congratulations Camilla Whittington

Camilla Whittington with sea horses University of Sydney
Viviparity and pregnancy has evolved numerous times and Camilla Whittington has set herself the ambitious task to seek similarities in gene expression that support pregnancy in sea horses, viviparous lizards and marsupial mammals. Her paper on the transcriptome of the brood pouch of a male sea horse was highlighted in a previous post.

Now I am delighted to report that Camilla has been awarded a Fondation L'Oréal Women in Science Fellowship. It is good that this award exists and especially encouraging that research in comparative biology of pregnancy has become known to a wider public through the publicity surrounding Camilla's Award.

Sea horses from Camilla's web site
To learn more about Camilla's Work visit her web site. For hard science you can read about the comparative genomics of hormone signalling in the chorioallantoic membrane here.

Monday, 10 October 2016

Did placental oxygen transfer support increased brain metabolism as humans evolved?

Cerebral blood flow in relation to age of fossil skulls
from 12 hominin species.
Reproduced from Seymour et al.  R S Open Science (CC-BY)
 
There is much interest in how increased brain size impacted on dimensions of the pelvis, birth weight and placentation as humans evolved (here). Increased brain growth in fetal life would seem to demand a greater placental blood flow to ensure an adequate oxygen supply. This may have led to increased invasiveness (discussed here) and indirectly to the risk of preeclampsia (here). 

But what if there was an increase not only in size but in the metabolic rate of the brain?

In a novel approach, Seymour, Bosiocic and Snelling attempted to assess cerebral blood flow in fossil adult hominins as a surrogate for oxygen supply to and consumption by the brain. As their starting point they measured the diameter of the carotid foramen in skulls of fossil hominins ranging from Australopithecus to archaic Homo sapiens (the carotid foramina carry the principal arteries supplying the brain). As can be seen in the figure, their main finding was that during hominin evolution cerebral blood flow increased disproportionately to brain size. The implication was that there was a progressive increase in the metabolic rate of the brain.

Of course this approach required some major assumptions (see below). But if the metabolic rate of the adult brain did increase successively as humans involved, so perhaps did that of the fetal brain. Here more is at play than the rate of blood flow to the brain. In adults the blood becomes fully saturated in the lungs, but that is not the case in fetal life. The oxygen content of the blood reaching the brain is dependent on placental function (reviewed here).

For Neanderthals there is enough data to conclude that brain size at birth did not differ from that of modern humans (here). But if human fetal brain had a higher metabolic rate than Neanderthal fetal brain, it might still have required a more efficient placenta.

Assumptions: The approach adopted by Seymour et al. relied on rearranging an equation for shear stress to isolate one of its determinants, blood flow rate. To accomplish this, shear stress must first be estimated using a scaling model that relates shear stress to body size. This is the weak point in the analysis, but the authors point to a previous study that verified the approach in primates and marsupials