Monday, December 31, 2012

Erik Jarvik - paleoichthyologist

Erik Jarvik (1907-1998)
Image Miguasha National Park
Anders Erik Vilhelm Jarvik (30 November 1907 – January 11, 1998) was a Swedish palaeozoologist who worked extensively on the sarcopterygian (or lobe-finned) fish Eusthenopteron.

In a career that spanned some 60 years, Jarvik produced some of the most detailed anatomical work on this fish, making it arguably the best known fossil vertebrate."
Erik Jarvik did his lifework in the Swedish Museum of Natural History in Stockholm starting in 1937 and becoming the professor and head of department in 1960. Jarvik has written extensively on sarcopterygian fossils and published two books on the subject
  • Théories de l'évolution des vertébrés reconsidérées à la lumière des récentes découvertes sur les vertébrés inférieurs. Masson, Paris. 1960.
  • Basic Structure and Evolution of Vertebrates, 2 Vols. Academic Press, London. 1980 

3D-modelling before computers

Erik Jarvik’s wax model showing the cavities and canals
inside the neurocranium of Eusthenopteron foordi
Image Miguasha National Park
Erik Jarvik conducted detailed anatomical studies of the cranium of Eusthenopteron foordi using a serial-section technique introduced by William Johnson Sollas and applied to fossil fishes by Erik Stensiö.

A fossil of limited external quality was sectioned by grinding off a thin section, photographing the grind-off end and repeat the process until the whole fossil was worked through. The internal structures would then show up on long series of photographs.

Working in the day before computer simulations, models was made by projecting reversal film on a board, and cut thin wax plates to match. The sticky wax plates could then be assembled to a 3 dimensional scaled up model of the skull, complete with internal structures such as nerve channels and other internal hollows rarely seen in fossils.

Further section to the cranium could easily be made by cutting the wax model at the desired angle. Due to the sticky nature of the wax used, a sectioned skull was put back together by simply pressing the two sections back together. This technique was also applied to the cranium of the porolepiform Glyptolepis groenlandica.

Important but passée
On the basis of his detailed studies of the heads of Eusthenopteron Jarvik proposed a theory on the evolution of tetrapods in which amphibious animals were classed into two groups, one related to salamanders and the other to frogs. According to Jarvik amphibian life evolved twice in prehistoric planet Earth.

Jarvik also studied the anatomy and relationships of lungfish which he held to be relatively primitive gnathostomes, possibly related to holocephalans, and of acanthodians, which he considered to be elasmobranchs rather than osteichthyans.

He made contributions to a number of classical problems in comparative anatomy, including

  • the origin of the vertebrates 
  • the origin of the pectoral and pelvic girdles 
  • the origin of paired fins
  • the homologies of the frontal and parietal bones in fishes and tetrapods 

Finally, Jarvik investigated the anatomy of Ichthyostega, resulting in a monograph with an extensive photographic documentation of the material collected in 1929-1955.

Jarvik, E. (1996). The Devonian tetrapod Ichthyostega. Fossils and Strata, 40, 1-213.
Some of Jarvik’s views did not accord with general opinion in vertebrate palaeontology   However, his anatomical studies of Eusthenopteron foordi laid the foundations for modern studies of the transition from fishes to tetrapods. 

Jarvik was a member of the Royal Swedish Academy of Sciences and French Academy of Sciences and Knight of the Order of Vasa. The lungfish Jarvikia and the osteolepiform Jarvikina are named after him.

Thursday, December 20, 2012

Third day Devonian

Tree-ferns in Nunniong, Australia
Image: Fir0002/Flagstaffotos
The earliest known trees appear in Devonian. The Late Devonian Wyazzit looked much like the unrelated tree-ferns in the photo and could grow 8 meters high.

The greening of land
"The Devonian period marks the beginning of extensive land colonization by plants. With large herbivorous land-animals not yet present, large forests could grow and shape the landscape.

Early Devonian plants did not have roots or leaves like the plants most common today and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall. By far the largest land organism was Prototaxites, the fruiting body of an enormous fungus that stood more than 8 meters tall, towering over the low, carpet-like vegetation.

By Middle Devonian, shrub-like forests of primitive plants existed: lycophytes, horsetails, ferns, and progymnosperms had evolved. Most of these plants had true roots and leaves, and many were quite tall.

In the Late Devonian, the tree-like ancestral fern Archaeopteris and the giant cladoxylopsid trees grew with true wood. (See also: lignin.) These are the oldest known trees of the world's first forests. The earliest known trees, from the genus Wattieza, appeared in the Late Devonian around 380 Ma.

By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion".

The 'greening' of the continents acted as a carbon dioxide sink, and atmospheric levels of this greenhouse gas may have dropped. This may have cooled the climate and led to a massive extinction event, Late Devonian extinction."

And God said, “Let the waters under the heaven be gathered together unto one place, and let the dry land appear”; and it was so. And God called the dry land Earth; and the gathering together of the waters called He Seas; and God saw that it was good.

And God said, “Let the earth bring forth grass, the herb yielding seed, and the fruit tree yielding fruit after his kind, whose seed is in itself, upon the earth”; and it was so. And the earth brought forth grass, and herb yielding seed after his kind, and the tree yielding fruit, whose seed was in itself, after his kind; and God saw that it was good.

And the evening and the morning were the third day.
Genesis 1:9-13 KJ21

1. Both texts specify the importance of plants that now begin to appear

2. Both texts list attributes of the various plant species

3. "Late Devonian Extinction"  -  "And the evening came" as boundary marks. Five major extinctions are known in the history of life on planet Earth. Genesis has six days of creation before the great Sabbath.