In pursuit of early mammals

A multituberculate (Catopsbaatar) about to fall prey

to a dinosaur (Saurornithoides)

This fascinating book is the autobiography of the eminent paleontologist Zofia Kielan-Jaworowska and an introduction to the early history of mammals.

Her descriptions of the Polish-Mongolian expeditions in 1963-71 recount the daunting logistics of working in the Gobi Desert 1000 km from Ulaanbaatar and transporting back fossils weighing several tons. The excitement of the fossil hunt is apparent: "I turned the block over and was left speechless! I had in my hands an almost complete, beautifully preserved skull of a small dinosaur." With this as a starting point we follow Zofia Kielan-Jaworowska's subsequent career, which included eight years on the faculty of Oslo University. In 2002 she returned to Gobi with her grandaughter Zosia; ten years on Zosia checked the references for the current volume. 

The second half of the book focuses on the evolution of mammals. It is restricted to the crown group that includes the living monotremes, marsupials and placentals and extinct groups with which they share a common ancestor. Arguably this includes the fossils of greatest interest to the general reader. For a more comprehensive treatment of the subject there is Mammals from the Age of Dinosaurs (2004) by Kielan-Jaworowska, Cifelli and Luo.

The new book contains potted biographies of many eminent vertebrate paleontologists, including some of the author's predecessors as well as recent stars such as Zhe-Xi Luo. It is richly illustrated with historical photos and line drawings of fossil mammals from the age of dinosaurs.

The spiny mouse

Spiny mouse (Acomys sp.) Wikimedia Commons

Despite its appearance the spiny mouse belongs to a different subfamily than the laboratory mouse. More importantly, it has a different reproductive strategy. Litter size is smaller and much of organ development occurs in utero resulting in the birth of precocial young. Thus it is a better model for late gestation in humans than the mouse, which has large litters and altricial young (see previous post).

At the same time, the laboratory species (Acomys cahirinus) is sufficiently close to mouse and rat that it is possible to develop primers based on the nucleotide sequences of mouse and rat genes (exemplified here). 

A group at Monash University has been using the spiny mouse in studies of fetal programming and sex-dependent effects of glucocorticoids on placental development (here). The spiny mouse holds great promise as a new model for placental and fetal development. Moreover, its potential for studies in a quite different area, tissue regeneration (here), means it is likely to be come more widely available as a laboratory animal.

Maureen Young 1915-2013



Maureen Young in Denmark 1989

Professor I. Maureen Young made an important contribution to our knowledge of amino acid transfer across the placenta and its role in fetal nutrition.

Maureen Young graduated from Bedford College for Women in 1938 and continued there as a demonstrator and assistant lecturer. During the Second World War the College evacuated to Cambridge where Maureen met Sir Joseph Barcroft. This was the start of a life long interest in fetal physiology (see her review of Barcroft's book here). In 1946 she moved to St. Thomas's Hospital Medical School. She was Professor of Perinatal Physiology there from 1976 until her retirement in 1982. She then settled at Toft in Cambridge.

Maureen was one of a generation of women who rose to the top in research often at great personal sacrifice. She wrote about some of the others in Women Physiologists (Portland Press 1993). At placenta meetings she was often to be found in the company of her American counterpart Elizabeth M. Ramsey (previous post).

Although international travel was eventually curtailed, Maureen continued to attend scientific meetings at Cambridge until well into her 95th year. She will be greatly missed. 

An ancient endogenous retrovirus

Classes of ERVs Wikimedia Commons

Endogenous retroviruses (ERVs) are genes of retroviral origin that have been incorporated into the genome. They include the syncytins expressed in the placentas of various mammals. Six are known so far and each represents a separate capture of a retroviral gene (see previous post). 

Now an ERV has been described that occurs in representatives of all four superordinal clades of placental mammal (here). In this case the gene is orthologous and thus represents a single integration event that must have predated the divergence of placental mammals in the Cretaceous (for possible dates see my recent post).

This work was published in a special issue of Phil Trans R Soc B on the theme "Paleovirology: insights from the genomic fossil record." Although sequence substitutions occur over time, an ERV is essentially a "fossil" record of a virus as it existed at the time of incorporation into the host genome. Thus the study of ERVs in various organisms (not just mammals) can provide insight into the long term history of viruses and virus-host interactions (see overview here).

In a historical overview of the field (here) Robin A. Weiss notes, "The advantage to the host of ERV protein expression is most dramatic in the evolution of the mammalian placenta." Incidentally, if you find the whole concept hard to swallow you are in good company as the early history of the field so clearly shows.

The Mammalian Ovary

Mossman and Duke 1973

It is now 40 years since publication of Harland Mossman's "other" book (previous post). It differs in many respects from the better known volume on fetal membranes. In particular it is richly illustrated with photomicrographs. Most of the specimens were in the collections of its authors. The Harland W. Mossman Collection is conserved at the University of Wisconsin Zoological Museum and there is a searchable data base (here).

Kenneth L. Duke was an alumnus of Brigham Young University at Provo, Utah. On retiring from Duke University he sent his collection there. Mercifully it has remained intact although it has not been catalogued. Much of the material will be from Duke's yearly field trips to The Great Basin but there is also material from South East Asia. There are placentas from some species including the Philippine colugo.

Comparative Morphology of the Mammalian Ovary takes as a starting point the red squirrel (Tamiasciurus hudsonicus) before dealing with a number of topics across the full range of mammals. For individual taxa the synoptic tables at the back of the book are unsurpassed as a source. The footnotes to these tables indicate where they rely on Duke's Collection now at BYU.

Disclosure: I am adjunct curator of The Harland W. Mossman Collection.    

Alternative views on the origin of placentals

The first placental mammal

From O'Leary et al. Science 2013; 339: 662-7

Reprinted with permission from AAAS

Hot on the heels of the controversy about crown mammals comes a dust up over crown placentals. This time in Science and in response to an important paper by O'Leary et al. that combined morphological and molecular data (see previous post). One conclusion in that paper (here) was that ordinal diversification of placentals occurred after the Cretaceous-Paleogene boundary (Explosive Model, cf. Wednesday's post).

This has elicited a response from an eminent group of molecular phylogeneticists (Springer et al. here). Their best estimate of the origin of placentals (here) posits a common ancestor 100 Mya with most orders arising in the Cretaceous (Long Fuse Model). They contend that that molecular phylogenies are more reliable than phenome-based (morphological) phylogenies even when they imply ghost lineages (i.e. with no fossil record).

In their rebuttal, O'Leary et al. (here) note that molecular phylogenetics has failed to resolve the basal split in Placentalia (see here) or to establish a sister taxon for primates. Thousands of nonplacental Cretaceous fossils are known so why are placentals absent? 

A seemingly weightier point made by Springer et al. is that ordinal diversification over a short time span in the early Paleocene implies high substitution rates. Maybe so respond O'Leary et al. Let's redo our tree but impose average substitution rates as a limitation. The result is to extend the origin of orders past the K-Pg boundary but only by some 6 My. That puts the common ancestor at 70 Mya which is still 30 My later than the molecular estimate.

New fossils support alternative views of mammalian evolution

Alternative interpretations of early mammalian history

Reproduced with permission by Macmillan Publisher Ltd.

from Cifelli and Davis (here) copyright 2013

Crown Mammalia are the extant monotremes, marsupials and placentals together with all extinct species with which they share a common ancestor. It is widely accepted that they include the multituberculates that became extinct in the Oligocene.

Two well preserved Jurassic fossils from another group of mammaliforms, the haramiyids, are described in the current issue of Nature. In one paper (here) the fossil form (Arboroharamiya jenkinsi) is found to group with the multituberculates and thus within Crown Mammalia. Since the haramyids have a deep fossil record this pushes the common ancestor of crown mammals back into the Late Triassic.

The second analysis (here) of a different fossil (Megaconus mammaliaformis) places haramiyids outside crown placentals, which thus can retain a Jurassic origin.

An accompanying commentary (here) notes that a Triassic origin is in accordance with the "long fuse" hypothesis of mammalian evolution whereas a Jurassic origin better corresponds to the "explosive" hypothesis. These authors tend to favour the conclusion based on Megaconus as it is a more complete fossil and the counterparts of the middle ear bones remain attached to the mandible, differing in this respect from extant mammals and multituberculates.