Leonard Ginsburg of the French Natural History Museum, published his first thesis on the extinction of the dinosaurs back in 1964, he believes that a gradual drop in world sea levels (marine regressions) led to disastrous climate changes for dinosaurs (1). Since then digs in western Canada and the U.S. have shown that 75 million years ago there were 30 species of dinosaurs. Five million years later there were 23, within two million years the number had fallen to 18, and so on down until the end of Cretaceous period when all the dinosaurs had died out.
Horner et al has shown that 70 million years ago marine transgressions caused habitat bottlenecking (2), this type of environmental stress or the changes in autonomic tone (AT) maybe reproduced in extant animals. Dinosaur paleoneurology has primarily focused on the central nervous system with the discovery of intact brain cases. The autonomic nervous system (ANS) has basically been overlooked because soft tissue readily decays upon death. AT is a significant deteriment of a species's physiological state and evidence for its changes over time can be detected from fossilized bone. ANS has two divisions; the sympathetic nervous system which speeds up heart rate (HR), narrows blood vessels and raises blood pressure; the parasympathetic nervous system slows HR and increases intestinal and glandular activity. The diagram to the right is an illistration of AT with respect to cardiac output (CO) where SV is stroke volume. Verrier has shown that adrenaline is released from the adrenal medulla causing peripheral constriction and hypertension in a model of behavioral stress (3). Continued stress will reset the baroreceptors resulting in hypertension. To the left is an actual recording of the heart rhythm during an infusion of the calcium channel antagonist verapamil which lowers blood pressure. The bottom trace is the blood pressure, if you compare the left side of the tracing to the right, you will see the resetting of the baroreceptors (4).
During the marine transgressions hypertension occured in dinosaurs, resulting in the loss of sexual ornamentation. The nasal boss on achelosaurus and pachyrhynosaurus are good examples of peripheral vascular constriction caused by the hypertension. Tyrannosaurids forearms are another example with the muscle atrophy and osteolysis. Lambeosaurines and centrosaurines went extinct after the last transgression (70My) probably because they were adapted to a high ST. During regressions increased sexual ornamentation occurs, with hypotension. Infact the most prevalent of Lancian (67My) dinosaurs, triceratops exhibits high peripheral vasodilatation (vagal tone). Dinosaurs were not able to adapted fast enough, unlike mammals with short gestation periods, to keep up with marine transgressions and regressions, therefore they went extinct.
It has been universally accepted that birds are direct descendants of theropod dinosaurs. Ruben et al have suggested, that the eariest stages in the derivation of the avian abdominal air sac system from a diaphragm-ventilating ancestor would have necessitated selection for a diaphragmic hernia in taxa transitional between theropods and birds (5). A diaphragmic hernia (DH), an opening in the diaphragm that permits abdominal organs to enter the pleural cavities and impinge upon normal development and functioning of thoracic organs, actually is quite common in domestic animals (6). In cats its been estimated to occur once in every 1000 live births, probably the result of an autosomal recessive gene (7). This condition arises during the early development (approximately 25 days old in cats) of the liver and heart. The prognosis depends on the degree of pulmonary hypoplasia, some patients remain clinically unrecognized for several years or are recognized only at necropsy (8,9). The avian abdominal air sac system and parabronchi are the result of a series of autosomal recessive defective genes and birds are direct descendants of dinosaurs.
The example above of a diaphragmic hernia shows the importance of developmental genetics in paleontology. But there are limitations in molecular genetics in paleontology, with the difficulty in the ability to extract biomolecules and the interpetation of those biomolecules (10) and the simularities between different genomes of organisms(11,12). Genetically speaking there were three major requirements for dinosaurs that were not met. 1) a stable enviroment, 2) a large gene pool, and 3) a short embryonic developmental (gestation) period. Nome of them were met, therefore they went extinct.
1) Ginsberg L; Les regressions marines et le probleme de renouvellement des faunes au cours des temps geologiques. Bull. Soc. Geol.Fr. 6:13-22, 1964
2) Horner JR, Varricchio DJ, Goodwin MB; Marine transgressions and the evolution of Cretaceous dinosaurs. Nature 358:59-61, 1992
3) Verrier RL; Autonomic substrates for arrythmias. 65-85 in Zipes DP, Rolands DJ (eds), Progress in Cardiology, 1988
4) Takahata O, Krolikowski JG,McCallum JB, Lathrop DA, Bosnjak ZJ; Effects of the optical isomers of verapamil on electrophysiological properties of the heart in conscious dogs. European Journal of Pharmacology 355:159-166, 1998
5) Jones TD, Ruben JA, Geist NR; Lung structure in theropod dinosaurs: implications for physiology and phylogeny. Journal of Vertebrate Paleontology 17(3):56A, 1997
6) Finn JP, Martin CL; Diaphragmatic pericardial hernia. J Small Animal Pract. 10:295, 1969
7) Atkins CE; Suspect congenital peritoneopericardial diaphragmatic hernia in an adult cat. JANMA 165:175, 1974
8) Baker GJ, Williams CSF; Diaphragmatic pericardial hernia in the dog. Vet. Rec. 78:578, 1966
9) Weitz J, Tilley LP, Moldoff D; Pericardiodiaphragmatic hernia in the dog. JANMA 173:1336, 1978
10) Higby Schweitzer M, Johnson C, Zocco TG, Horner JR,Starkey JR; Preservation of biomolecules in cancellous bone of Tyrannosaurus rex. Journal of Vertebrate Paleontology 17(2):349-359, 1997
11) The Caenorhabditis elegans Sequencing Consortium. Genome sequence of the nematode C. elegans : a platform for investigating biology. Science 282:2012-2018, 1998
12) Venter JC, et. al. The sequence of the human genome. Science 291:5507, 2001
This page and all contents are Copyright (C) 2001 John Krolikowski-All Rights ReservedLast Updated:7/19/2001