Paleontology
The notion of evolution draws attention to fossil evidence (Figure 1.5). George Cuvier (1769-1832) was among the first to extensively describe extinct species, interpreting the evidence as arising from a series of catastrophic events that had led to the extinction of some
Figure 1.4 A fossil glyptodont, an extinct mammal from North and South America, and a living armadillo. There are more than 20 living species of armadillos distributed across the Americas. Taxonomically these animals are classified as edentates (Order Edentata), together with tree sloths and anteaters. The name “edentata” refers to the absence of teeth—although armadillos do have molars.
Source: Photos from Creative Commons, https://search.creativecommons.org; glyptodont by Ryan Somma, license CC BY 2.0; armadillo by mbarrison, license CC BY-SA 2.0.
Figure 1.5 Some fossil remains can be extremely common, such as these sea urchins, bivalves, and ammonites collected in central Texas, in late Cretaceous rocks.
Source: Photo by the author.
species. Evidence of evolution in the fossil record is found in the sequential appearance of the various animal groups.
Among vertebrates, for example, the oldest known fossil fish appeared in Silurian rocks from China and lived some 419 million years ago (Ma) (Zhu et al., 2013). Rocks from that period or older yield no evidence of land vertebrates. Tetrapods appeared some 100 million years later, during the Devonian, which, in turn, provides no evidence of birds or mammals. It is only during the Jurassic, some 200 Ma, that birds and mammals appear as fossils, and only 65 Ma that mammals started to diverge into the living orders we know today. This sequential order is also replicated within more restricted taxonomic groups. In horses, for example, small body size and multiple digits are basal traits; derived traits, such as large body size and reduced digit number, appeared in more recent fossils. In primates there is also a similar increase in body size, with the largest species (apes and hominins) evolving relatively more recently than the smallest species (lemurs).
Some fossil specimens provide compelling evidence of the genealogical links among different groups of animals. For example, Archaeopteryx lithographica, a fossil bird from the Jurassic discovered in Germany in 1862, exhibits a mixture of reptilian traits (bony tail, teeth) and avian traits (feathers, wings). Such specimens are transitional forms of the type predicted by Darwin’s notion of species continuity (Figure 1.6). In some cases, fossils actually provide molecular material, as it happened with some hominin specimens (e.g., Neanderthal, Den-isovans, and early Homo sapiens; see Chapter 12). Such amazing discoveries open the possibility of combining paleontological and molecular evidence in a manner that would have been considered impossible just a few decades ago. Studies of ancient DNA must control for the possibility of potential contamination of the samples with modern DNA.
Figure 1.6 Archaeopterix lithographica, a fossil bird from the Jurassic of Germany, is an example of a transitional form with reptilian and avian traits. Like reptiles, and unlike any living bird, Archaeopterix had a bony tail and teeth. Like living birds, Archaeopterix had feathers and the anterior limbs were modified as wings.
Source: Photo reproduced with permission from the Museum fur Naturkunde, Berlin, Germany.
