Animals (continued)

3. Land Vertebrates

3.1 Introduction

The following table, taken from a paper by Ezaz et al. in Current Biology16, 5 Sep 2006, describes the Phylogeny and the Sex Determination Systems of Vertebrates.

Sex determination systems are explained in the page Sex Chromosomes in Birds and Mammals at this site, which is actually an edited copy of a page at the site www.euronet.nl/users/hnl, where a great deal of information is given. The present page deals mainly with the evolution of tetrapods (land vertebrates); detailed information about each class/order/suborder may be found at the links in the table above.

Tetrapods are traditionally grouped into the classes AMPHIBIA (amphibians), REPTILIA (reptiles), AVES (birds) and MAMMALIA (mammals). Reptiles fall into four living orders: CROCODILIA, SPHENODONTIA (tuataras), SQUAMATA (snakes and lizards) and TESTIDUNES (turtles). Eutheria, marsupials and monotremes are mammals.

We see from the table that BIRDS and CROCODILIANS are closer to each other than to SQUAMATA. Squamata or scaled reptiles is the largest order of reptiles, including lizards and snakes. Their skins bear horny scales or shields. They also possess quadrate bones, making it possible to move the upper jaw relative to the braincase. The TUATARA is found only in New Zealand. It now lives only on predator free outlying islands, but can be seen on the mainland in "protective custody." BIRDS are now believed to be direct descendents of the dinosaurs. The taxonomical progression, as given by tolweb, leading to birds, is:

Amniota > Diapsida > Archosauromorpha > Archosauria > Dinosauria > Theropoda > Coelurosauria > Aves

The base clade AMNIOTA splits up immediately into SYNAPSIDA (mammals and extinct related types) and DIAPSIDA, which includes birds and present day reptiles excluding turtles. The clade TESTUDINES (turtles) is also believed to branch off at this point. Diapsida is the smallest clade containing the Ichthyosauria and ARCHOSAURIA the smallest containing the Pterosauria. Crocodiles also branch off at archosauria.

3.2 The Geological Succession

Further information can be found in the dmoz directory page on EARTH SCIENCES. The page rainbow.ldeo.columbia.edu/courses/v1001/geotime2.html is particularly relevant to this section.

Palaeontology, the study of fossils, is intimately connected with stratigraphy, the branch of geology dealing with rock layers and layers (stratification). Stratigraphy was first developed by Nicolaus Steno (1638 - 1686), who postulated the following four principles in 1669:

• Principle of superposition of strata. This was first outlined by Avicenna, who formulated it as follows:

  It is also possible that the sea may have happened to flow little by little over the land consisting of both plain and mountain, and then have ebbed away from it. ... It is possible that each time the land was exposed by the ebbing of the sea a layer was left, since we see that some mountains appear to have been piled up layer by layer, and it is therefore likely that the clay form which they were formed was itself at one time arranged in layers. One layer of clay was formed first, then at a different period, a further was formed and piled, upon the first, and so on. Over each layer there spread a substance of different material, which formed a partition between it and the next layer, but when petrification took place something occurred to the partition which caused it to break up and disintegrate from between the layers (possibly referring to unconformity). ... As to the beginning of the sea, its clay is either sedimentary or primeval, the latter not being sedimentary. It is probable that the sedimentary clay was formed by the disentegration of the strata of mountains. Such is the formation of mountains.

  See www.muslimheritage.com/uploads/ibnsina.pdf.

• Principle of initial horizontality of strata. Sediments are deposited at their angle of repose which, for most types of sediment, is essentially horizontal.
• Principle of lateral continuity. Layers of sediment initially extend laterally in all directions unless obstructed by a physical object or topography.
• Principle of cross cutting relationships. Whatever cuts across or intrudes into the layers of strata is younger then the layers of strata.

Steno pointed out that glossopetrae or "tongue stones," found scattered along some layers in the mountains around Tuscany, were in fact the teeth of sharks that had lived when the mountains were under water. However, the principles of palaeontology were developed by William Smith (1768 - 1839), who identified strata by fossils in the period 1816 - 19.

This led gradually to the construction of a general table of rock groups, called the geological succession, which may also be treated as a geological timetable. A summary of this table is given below.

Abbrev. mya = "million years ago"

The dating of the geological strata was finally established by radiometric methods in the beginning of the twentieth century. The basic chronology is given below:

Further information on this subject can be found at www.talkorigins.org/faqs/geohist.html. The development of atomic theory which preceeded the discovery of radioactivity is discussed in the page "Chemistry in the Nineteenth Century" at this site.

Not all rock strata are fossiliferous; fossils are only found in some sedimentary and metamorphic rocks. Fossil types are a sure guide to the nature of the deposition of the rocks in which they are found. coal beds, beds containing fossil snails, and amber are three clear indications of the existence of some types of land surface.

Civil engineers occasionally use fossils to determine the exact nature of a certain strata with special reference to other beds. More generally, however, the works of the engineer will reveal sections of fossiliferous strata that would otherwise remain hidden to the geologist. Although not strictly a fossil remain, the skull of the now famous Rhodesian man must be mentioned in this connection. It was discovered during excavation work of the Broken Hill Mining Company in South Africa. This is mentioned in Chapter 10:A Short History of the World by H. G. Wells. More recently, several fossils were unearthed during the widening of the Panama Canal. An account of this is given in my Windows Live blog posting ON THE PANAMA CANAL EXPANSION. William Smith too was a canal engineer. While working on Somerset coal, he was asked by Lady Elizabeth Jones, a local coal owner, to survey the Mearns Pit at High Littleton. It was there that he noticed the pattern of rock stratification. See

www.geog.port.ac.uk/webmap/hantscat/html/smith4.htm

for details. The act for the Somerset Coal Canal was passed in 1794 and William Smith was its surveyor.

The following image gives some chronological details of the evolution of land vertebrates.

Source: Systematic Biology, Volume 56, Issue 3, pp, 369 - 388

Notice that the last column gives further subdivisions of geological time. (Information about this is given in the Palaeos.org pages Carboniferous and Permian.) The second column gives the estimated times of evolution of the amphibia and the amniotes. The first gives some information about the evolution of land vertebrates. The ichthyostega were the first Devonian tetrapods to be found and described (Säve-Söderbergh 1932). The lepospondyls consist of four to six groups of mostly small aquatic tetrapods.

Reproduced below for reference are the definitions in the paper just cited:

3.3 Amniotes

The embryos of reptiles, birds and mammals are covered by some structures which are outside the embryo and which do not take part in the formation of the organs of the actual embryo. Such structures are called extra-embryonic membranes. The amnion, chorion, allantois, yolk-sac are all extra-embryonic membranes. They are formed from the germinal layers outside the embryo. When the embryo develops fully, i.e. when it hatches or is born, as the case may be, the extra-embryonic membranes get separated from the young one.

From "Textbook of Vertebrate Embryology" by N.N.Majumdar

The extra-enbryonic membranes are the following:

1. Amnion Non-vascularised extra-embryonic membrane of the amniote embryo in the form of an umbrella over the embryo.
2. Chorion (or serosa) The membrane formed by the outer layer of the amniote head, lateral and tail folds. It has two layers of cells composed of the ectoderm outside and the somatic endoderm inside.
3. Yolk sac The endodermal and splanchnic mesodermal covering in the shape of a spherical sac over the yolk.
4. Allantois Vascularised extra-embryonic membrane, usually in the form of a balloon-like projection from the hind gut of the amniote embryo.

The next three images (all from Majumdar loc. cit.) illustrate the formation of the extra-embryonic membranes in the case of the chick.

(A) 30 hours of incubation	(B) 50 hours after incubation
(E) 70 -75 hours of incubation	(F) 4th day

----x-----x----

----x-----x----

In the case of mammals, eggs are not laid but the embryo develops in the body of the mother. There is now one more structure, the placenta, involved. This, strictly speaking, is not a membrane, because it is not a sheet of tissue, but once more is formed outside the body of the embryo.

The placenta facilitates the transfer of oxygen and nutrition from the mother's blood to that of the foetus, and transfer of waste products from the foetal blood to the mother's.

The next image (also from Majumdar loc. cit.) illustrates the development of the human foetus.

According to the page Early Researchers & Finds at jogginsfossilcliffs.net, the earlist known amniote in the fossil record is the hylonomus lyelli, discovered in 1859 by J. William Dawson and named in honour of Sir Charles Lyell. A lot of publicity has been given to the discovery of casineria kiddi (Paton, Smithson & Clack 1999), but a complete skeleton has yet to be found. Casineria is believed to be the most primitive reptile in the Archosauromorpha (crocodiles, birds, etc.).

A rather old paper dealing with amniote cladistics, giving much detailed information, is "Amniote Pylogeny and the Importance of Fossils" by Jacques Gauthier, Arnold G. Kluge, and Timothy Rowe [Cladistics (1988)4: 105 -209].

3.4 Primitive Reptile Types

Reptiles and higher vertebrates evolved from early land amphibians called labyrinthodonts. The labyrinthodonts had no openings in the skull except for the nostrils and eyes. The reptiles that evolved from these early creatures have been grouped according to differences in the skull. There are four groups classified according to the number and location of extra holes (called apsids or fenestrae) in the back part of the skull.

These can be seem in the next image:

Source: http://www.all-about-reptiles.com/evolution-of-reptiles.html

(See also the page www2.fiu.edu/~longoria/gly1101?Mz-reptiles.htm for more detailed explanations.)

3.5 Richard Owen and the Origin of the word Dinosaur

Sir Richard Owen (1804 - 1892), a British comparative anatomist, coined the term dinosauria. He noticed that a group of fossils (including megalosaurus, iguanodon and hylaeosaurus) had certain characteristics in common, including:

• Column-like legs rather than the sprawling legs of other reptiles (see image below):

  

• Five fused vertebrae fused to the pelvic girdle.

Owen proposed this new name in an article published in the "Proceedings of the British Association for the Advancement of Science" in 1842.

3.6 Evolution to Mammals

The second page mentioned in the previous paragraph refers to the Geocities page CapeCanaveral - The Therapsid Mammal Transitional Series, which is now available at the Internet Archive Wayback Machine.

The next image is a cladogram from the paper of J. GAUTHIER, A.G. KLUGE & T. ROWE (1988) cited in Section 3.3.

As we see, the cladistic (or evolutionary) progression leading to mammalia is

Synapsida > Sphenacondontia > Therapsida > Eutheriodontia > Cynodontia > Mammaliamorpha > Mammaliaformes

We now give the most important definitions.

The first known definite monotreme, Steropodon Galmani (early Cretaceous), was discovered in 1895. There are several primitive North American placental mammals dating beck to the late Cretaceous.

Finally, we give illustrations of some extinct animals on the evolutionary line towards mammalia.

DIMETRODON Synapsida	TETRACERATOPS Therapsida
DIADEMODON Cynodontia	MORGANUCODON Mammaliaformes

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