Rock Types:
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Stratigraphy: The interpretation of the sedimentary record
Sediments are typically laid down in basins (oceans, seas, lakes, floodplains, etc) where they typically form laterally continuous layers. The physical relationships of these layers enable us to determine the relative sequence in which sediments were laid down, using the Principles of Stratigraphy. Some of these were suggested as early as 1027 by Ibn Sina (Avicenna), but were formally codified by Nicholas Steno (1638-1686) and James Hutton (1726-1797). |
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Biotratigraphy: Of course, sedimentary rocks are deposited on the Earth's surface, in environments where Life lives. Consequently, fossils, the remains of living things, tend to be incorporated in them. In 1796 William Smith, a British civil engineer, added a fourth principle: Faunal succession, noting that different sedimentary rock units typically each contained distinct characteristic groups of fossil organisms - biostratigraphic fingerprints. Smith used these associations to construct the first technically competent geologic map, showing the relative ages of rock units distributed over a broad area. By this means it became possible to identify rocks of similar age even if they were on separate continents.
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So fossils tell us something about the age of rocks, but what do rocks tell us about the age of fossils? Prior to the 20th century researchers really didn't have any idea of the numeric ages of the rocks they studied. That changed with the discovery of:
Other lines of evidence later strengthened these results, so that the rock record provides multiple independent methods of establishing rock ages. These enable us to infer the chronology of the history of Life, as preverved in the rocks.
Fossils and Paleontology:
Fossils: Are the record of past life incorporated into the rock record. Paleontology is the study of past life. We encounter fossils in sedimentary rocks because, unlike other rocks, sedimentary rocks form where life lives.
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The History of Life in Five Minutes: What do the fossils tell us? A thumbnail chronology of life:
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Birds:
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Archaeopteryx - "The first 'bird'" (Late Jurassic -
~150 Ma.) When first discovered, in the 1860s, this creature was the only known feathered fossil. Thus, for over a century it has been a fundamental benchmark in paleontology - "the first bird." When it was first described, it was separated from other fossil creatures by a huge gulf. In the last 40 years, as more specimens of Archaeopteryx have been found and studied, and as our knowledge of small theropod (meat-eating bipedal) dinosaurs has improved, Archaeopteryx has come to seem less and less unique, as:
Thanks to the exceptional preservation conditions of rock units like the Yixian Formation of Liaoning Province, China, we now know that feather-like fuzz and proper feathers were widely distributed among smaller members of the theropods (the meat eating dinosaurs) like Caudipteryx. Indeed, we now know several small theropods that broadly resemble Archaeopteryx. Together, they form the base of the group Eumaniraptora, the theropods among whom feathers became functionally coupled with aerodynamics. Basal members of the major eumaniraptoran groups were broadly similar:
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 The Berlin Archaeopteryx 1881, from Linda Hall Library of Science, Engineering, and Technology
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But could any of them fly? Their forelimb mechanics make this much clear: They could definitely flap. Biomechanically, Archaeopteryx is considered to be at or near the threshold of flight. Its wings are very short, its breast-bone is not large enough to serve as the origin for powerful flight muscles, and its skeleton was not strengthened and fused in the manner of modern birds. If it could fly, it did so weakly and over short distances. So why did it waste metabolic energy growing and supporting big feathered forelimbs? Indeed, what did Caudipteryx do with its dinky "wings?"
Traditionally, paleontologists have considered two hypotheses for the origin of bird flight:
The discovery of creatures like Caudipteryx with its dinky feathered arms added color to this debate.
Note that these birds are NOT climbing in the typical sense: they are literally running up the sides of trees. Dial and his team studied the ontogenetic (growth) changes in the ability for birds to use this behavior, determining that WAIR is useful even in individuals with little dinky wings useless for flight. Sound familiar? We finally have a compelling evolutionary scenario for the evolution of wings from small theropod forelimbs.
Just this year, Denver Fowler of the Museum of the Rockies added a complimentary hypothesis, that flapping was used in flightless predatory eunamiraptorans like Velociraptor in order to stabilize the body on top of struggling prey, like modern birds of prey do.
Oddly, of the two major Eunamiraptoran groups, only Avialae stepped across the threshold of proper flight while deinonychosaurs backed away from it to become large flightless predators.
| Enantiornithes (Early - Latest Cretaceous): The most diverse and abundant group of birds of their time. Most were toothed. Encompassing a wide range of ecologies and sizes. |
 Enantiornis from Pavel Riha
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Ichthyornis (Late Cretaceous): A sea "bird" ecologically similar to living sea birds. This creature shared with proper birds, a keeled
breast-bone to support more powerful wing muscles. Even so, it retained primitive features such as teeth |
 Ichthyornis
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AVES (Late Cretaceous - Recent): The last common ancestor of all living birds and all of its descendants. Distinguished by:
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 Prespyornis from Critters Pixel Shack
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Derived features of living whales:
These are remarkable adaptations, but all the more so when one considers the starting point for this evolutionary trend: a primitive even-toed ungulate. Today, we think of these creatures as swift herbivores, but during the early Cenozoic Era (age of mammals), artiodactyls experimented with a variety of life styles including:
 The mesonychid Andrewsarchus from Bluelion.org
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 The cetacean Pakicetus from Taxonomy, Systematics, and Bioinformatics at the University of Glasgow
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The "whippo" hypothesis carries an interesting implication: Whales are not merely related to artiodactyls, whales ARE artiodactyls.
Landmarks in cetacean evolution: Watch for the following trends:
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Pakicetus: The best known primitive cetacean (Eocene epoch - Paleogene) Shares with more derived cetaceans an elongate snout in front of the nostrils, with incisors and canines arrayed in a V when viewed from below. Incisors and canines are simple cones, while post-canine teeth remain complex (in typical mammalian fashion).
Pakicetus is primarily known from the skull. Its lack of aquatic specializations in the ear moves most current researchers to regard it as primarily terrestrial (as below), although it may have hunted in shallow water or near the water's edge. |
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Ambulocetus: Roughly contemporaneous with Pakicetus (Eocene epoch - Cenozoic), Ambulocetus provides post-cranial remains that shed light on its locomotor adaptations. Ambulocetus remains are found in near-shore marine and estuarine deposits.
 AmbulocetusThe shape of its skull is reminiscent of a crocodylian, but its manner of axial swimming, involving both the vertebral column and the hindlimbs, is reminiscent of an otter. Perhaps it could also prey on land animals. |
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Rhodocetus: (Eocene epoch - Paleogene), Rhodocetus is also a limb and trunk propelled otter-style swimmer. It is slightly derived in the elongation of the trunk and the retraction of the nostrils compared to Ambulocetus.
 Rhodocetus
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Of particular interest is its well-preserved ankle. This shows two items of note:
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Basilosaurus: (Late Eocene epoch - Paleogene) By the end of the Eocene Epoch, profound changes in the locomotion and paleobiology of whales had occurred, as indicated by Basilosaurus (originally mistaken for a reptile, hence the misleading name.)
 Basilosaurus
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Of note:
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 Basilosaurus from Digital Designs
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Dorudon: Roughly contemporaneous with Basilosaurus Late (Eocene epoch - Paleogene), Dorudon approaches the biomechanical condition of living whales in:
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 Dorudon
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By its nature, the fossil record will always consist mostly of gaps. The wonder and glory is the real strength of the trend, over the last two-three decades, toward the narrowing of those gaps. In the case of whales, the emerging evidence is so good that it was cited by U.C. Berkeley paleontologist Kevin Padian as a case study of a compelling evolutionary story - a "poster-child" of evolution. Similar citations could be made to the developing stories of:
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