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Home , Albertosaurus, Allosaurus, Avetheropoda, Avialae, Carnosauria, Cetiosauridae

Chasing and Around the Tree of Life: Classifying 4 Thomas R. Holtz, Jr. Department of Geology Thomas R. Holtz, University of , College Park Jr. is a paleontologist in the Department of Geology at e owe the word “dinosaur” and its formal version Dinosauria, the University of meaning “fearfully great ,” to Sir . In 1842 Maryland, College Park. he published a paper that described the of Great He received his Ph.D. in WBritain, and he noted that three were geology and geophysics at sufficiently different from all other reptiles to warrant a particular group name of their own: . His carnivorous , herbivorous Iguan- The methods of primary research interests odon, and armored .1 In so doing, are the and Owen began the study of classifying dinosaurs. cladistic analysis adaptations of theropod Classification facilitates conversation. In or- continue to hold dinosaurs, especially the der to talk about anything, we have to have (tyrant names and labels: words that refer to items we great promise for dinosaurs); the are discussing. This is true whether we are talk- ing about sports teams, flavors of ice cream, our understanding ecomorphology of emotions, or dinosaurs. Furthermore, we group ; and the effect of these items into larger categories according to of the evolution of plate tectonics on different rationales: teams by sport, league, or the different groups terrestrial hometown; flavors of ice cream into fruits and distributions. He is non-fruits; emotions into “bad” (such as anger of dinosaurs. Director of the College and hatred) versus “good” (such as happiness and love); and so forth. In other words, we look Tom Holtz Park Scholars Earth, Life & for a (a system of names) and a Time program, a two- scheme of classification. honors program for undergraduates interested Classification Schemes in natural history. Dinosaurs, as , are given names and are classified in the same way that all organisms are named and classified. Prior to the 1700s there was no single set of rules of taxonomy used by scientists. Instead, different cultures and dif- ferent individuals in each culture organized animals, plants, fungi, and other organisms into particular schemes based on different attributes, such as the usefulness of the organism to , the danger it posed to humans, or its attractiveness. Carl von Linné, better known by the Latin form of his name, Linnaeus, developed the basic set of rules of biological nomenclature used by scientists since the mid-1700s. Linnaeus observed that the diversity of living things could be organized into a Natural System2, based on the features

31 D inosaurs:A Time and Place

present or absent in the physical form and on vive and perhaps diverge into more , or the behavior of the organisms. Linnaeus’ sys- alternatively fail to survive and become extinct. tem was universal. It could be applied to all or- Darwin realized that this pattern of diver- ganisms—plants, animals, fungi, and even gence of lineages through time was the reason bacteria. It was also international. The system that naturalists could uncover a nested hierar- established a set of names in Latin or Greek, or chical pattern to groups of organisms. For ex- rendered into a Latinate form, to be used by ample, lions and tigers (Panthera leo and scientists, regardless of their native tongue. Panthera tigris, to give the formal names of these taxa) are more similar to each other than Nested Hierarchies either is to grizzly bears (Ursus arctos), because The Linnaean system was organized as a nested lions and tigers share a more recent common hierarchy—a set of categories within larger cat- ancestor with each other than they do with egories within larger categories, like boxes bears. Darwin recognized that the reason for within boxes within boxes. Each named group, the hierarchical of life was the tree-like or (plural taxa), is a unit of biological di- structure of the history of life. He envisioned versity. Small units, like the species Tyrannosau- the stems of the tree representing common an- rus rex, are grouped into larger units, such as cestors in the past whose descendants branched the Tyrannosaurus, which are grouped into different lineages, some going extinct, oth- into even larger units, for example ers surviving and perhaps developing addi- Tyrannosauridae, , , and tional branches; and he saw all evolving new Dinosauria. and distinctive features in response to selection Linnaeus himself wrote, at least in his early from the world around them. Closely related works, that he considered species to be fixed; forms had diverged more recently from a com- that is, species did not change into other spe- mon ancestral population, while distantly re- cies. To him, the nested hierarchy was a useful lated forms represented branches that had split descriptor of the diversity of life because it off further down the Tree of Life. reflected the organized mind of the Creator. A In The Origin of Species Darwin proposed century later in his 1859 masterpiece The Ori- that the Linnaean system would have to be gin of Species,3 recognized the modified to recognize that the organizing prin- underlying reason for this pattern—common ciple of taxonomy is ultimately “propinquity of descent with modification. Darwin discovered descent”—that is, patterns of common ances- that organisms evolve in response to selection try. Darwin envisioned a method of of some variations in a population relative to classification where “more closely related to” the other variations. His idea of evolution by meant “shared a more recent common ancestor is sometimes over-simplified with,” rather than simply “is more similar in as changes in a single lineage through time, but appearance to.” Darwin recognized in The Ori- he also recognized it as responsible for larger gin that similarity might not always be a reli- patterns. Specifically, more than one set of able indicator of shared ancestry. There are variations in an ancestral population might examples of anatomical or behavioral features preferentially survive relative to the rest of their evolving independently, also called conver- kin. Over time different sets of variations gently, in separate branches of the Tree of Life, (physical or behavioral attributes) would be se- particularly if those features were responses to lected for in the two different subpopulations. similar environmental conditions. For ex- Eventually, these two subpopulations would be ample, animals that live in aquatic environ- so different from each other that they would ments tend to evolve streamlined shapes, not be able to interbreed, and would therefore regardless of common ancestry. Therefore, he represent new species. Thus, Darwin recog- cautioned that new approaches to classification nized a mechanism by which a single common should take a look at many characters in com- ancestral population could give rise to two or bination, in the hope that the actual historical more new species, which themselves could sur- pattern of ancestry will show up.

32 D inosaurs: The Behind the Stories Chapter 4—Classifying Dinosaurs

The Method of primitive features, unique features, shared de- rived features, and convergent features. Some After several attempts over the intervening features are present in all the taxa being stud- decades to search for a new approach to classifi- ied. Fig. 1 shows that in a set of four carnivo- cation that would reflect such patterns of ances- rous dinosaurs (, Deinonychus, try, a German entomologist developed the , and Tyrannosaurus) all had three organizing scheme used by most biologists to- 4, 5 features in common: (1) a hinge in the middle day. This entomologist, Willi Hennig, recog- of the lower jaw, (2) a wishbone, and (3) nized that it would be impossible to know every bipedality. Since they shared these features, single detail of the Tree of Life. Most individual then presumably the same features would be animals and plants are eaten or decay before found in the common ancestor of these four they can possibly be fossilized; indeed, many dinosaurs, and hence they are considered to be species of organisms will never be preserved in primitive (or ancestral) features. Alternatively, Fig. 1. the fossil record because they did not live in en- the most recent common ancestor of Allosau- Four carnivorous vironments where they would be likely to be rus, Deinonychus, Albertosaurus, and Tyranno- dinosaurs, and buried, preserved, fossilized, and later discovered saurus may have lacked a hinged jaw, lacked a by paleontologists. However, Hennig recognized wishbone, and walked on all fours, and in observations of that the shape of the Tree of Life would reflect which case each of these four dinosaurs evolved their features.

Allosaurus Deinonychus Albertosaurus Tyrannosaurus 1) Hinge in lower jaw Yes Yes Yes Yes 2) Wishbone Yes Yes Yes Yes 3) Bipedal Yes Yes Yes Yes 4) Retractable sickle No Yes No No 5) Backwards-pointing No Yes No No 6) Number of fingers 3 3 2 2 7) Third metatarsal in foot Unpinched Unpinched Pinched Pinched 8) Astragalus (ankle ) Short Tall Tall Tall 9) Tip of Expanded Pointed Pointed Pointed relative recency of common ancestry and this the features independently. However, that ancestry can be approximated by the distribu- would require more evolutionary changes than tion of features among the organisms that are the more straightforward, simpler explanation available. Hennig’s method became known as that they had all three features in common. cladistics (from , or “branch”), because it is Primitive features reveal that the taxa are re- primarily concerned with recovering the branch- lated at some level, but they don’t help us re- ing order of common ancestry. The method of solve who is more closely related to whom. cladistics is the search for the simplest distribu- Unique features are those found in only tion of derived features to approximate the his- one of the taxa being examined. For example, torical branching pattern of the Tree of Life. among the four dinosaurs considered, two fea- tures are found only in Deinonychus: (4) re- Different Types of Features tractable sickle claw on the foot and (5) a Hennig noted that the features found in a set backward-pointing pubis (Fig. 1). These fea- of organisms fall in four general categories: tures must have evolved after the ancestor of

D inosaurs: The Science Behind the Stories 33 D inosaurs:A Time and Place

Deinonychus split from all the other meat-eat- the first of all the forms in this study to branch ing dinosaurs in this study. They may be help- off from the others. Thus, the features found in ful in recognizing Deinonychus, but they don’t Allosaurus represent the ancestral condition for help us to determine which of the remaining the four taxa in this analysis. However, it is im- three dinosaurs was the closest relative of portant to understand that we do not regard Deinonychus. It should be noted that the use of Allosaurus as the actual ancestor of any of the “unique” or “primitive” for these features ap- dinosaurs in question! plies only to this particular analysis. For ex- Of the remaining three, Deinonychus, ample, also had a retractable sickle Albertosaurus, and Tyrannosaurus, it is likely claw and a backward-pointing pubis. Had it that one pair of these dinosaurs had a more re- been included in the present study, these fea- cent common ancestor than either did with the tures would be shared between Velociraptor and third. There are in fact three possibilities for Deinonychus, rather than limited to the latter. this situation: The remaining features of the original set 1) Deinonychus and Tyrannosaurus shared a are found in more than one, but not all, of the more recent common ancestor with each dinosaurs being studied. These features evolved other than with Albertosaurus (Fig. 2A); after the ancestors of the four dinosaurs in this study began to diverge from a common ances- 2) Deinonychus and Albertosaurus shared a tor, and are called derived features. Hennig rec- more recent common ancestor with each ognized that derived features potentially serve other than with Tyrannosaurus (Fig. 2B); or as clues to help discover the branching pattern 3) Tyrannosaurus and Albertosaurus shared a of the Tree of Life, because they evolved on a more recent common ancestor with each lineage leading to some, but not all, of the taxa other than with Deinonychus (Fig. 2C) being studied. Hennig also recognized that We do not know ahead of time which of there are two subsets of derived characters. these three possibilities is correct, but we can Some features might be convergent. These con- construct our hypothesis based upon the evi- vergent features would not help us discover dence. The criterion for determining which patterns of common ancestry because they pairing is most likely is simplicity. All other were evolved independently. The other fea- things being equal, we choose the simplest an- tures, though, may be shared derived features: swer. This criterion, known more formally as evolutionary novelties inherited from a com- parsimony, is a standard principle in science. mon ancestor. Hennig understood that by dis- When multiple possible explanations exist, the covering the pattern of shared derived features, one that requires the fewest assumptions is we might approximate the branching order of likely to be more nearly correct. the lineages leading to the creatures we are The three possibilities can be represented studying. as branching diagrams called (Fig. 2). A is the graphic representation Reconstructing the Tree of Life of a hypothesis of the relationships between Several features in this example represent de- taxa. Unlike traditional evolutionary rived, but not unique characters (Fig. 1). These tree drawings, in which the vertical lines repre- include (6) a two-fingered (rather than three- sented the actual group of reproducing organ- fingered) hand, (7) a pinched third metatarsal isms through time, the lines (branches) and the in the foot, (8) a tall astragalus ankle bone, and nodes joining the branches are just place hold- (9) a pointed ischium. None of these evolu- ers to represent common ancestry. For ex- tionary novelties occurs in Allosaurus (and in- ample, in Fig. 2C, there is a single node deed are lacking in meat-eaters more primitive marked “a” that represents the shared ancestry than Allosaurus, such as , Ceratosau- of Albertosaurus and Tyrannosaurus. The next rus, and ). This observation suggests node towards the base of this tree, marked “b,” that the lineage leading to Allosaurus represents represents that there is a common ancestor

34 D inosaurs: The Science Behind the Stories Chapter 4—Classifying Dinosaurs

shared by Deinonychus and the lineage leading to Alberto- AllosaurusAlbertosaurus Deinonychus Tyrannosaurus saurus and Tyrannosaurus. 5 7 7 4 6 And finally, there is a node 6 “c” that represents a common 11 Evolutionary Changes 9 ancestor shared by Allosaurus 8 3 and all remaining forms. 2 1 A

Choosing the AllosaurusTyrannosaurus Albertosaurus Deinonychus 7 reversed 6 reversed Best Hypothesis 5 As explained, only shared de- 4 9 8 11 Evolutionary Changes rived features can actually in- 7 6 form us about the pattern of 3 2 shared ancestral relationships. 1 B For any given set of taxa, more than one solution is AllosaurusDeinonychus Albertosaurus Tyrannosaurus possible (that is, more than 5 a one possible cladogram), each 4 7 b 6 representing a different hy- 9 Evolutionary Changes 9 pothesized relationship. By c 8 counting the number of evolu- 3 2 C tionary changes required to 1 produce each of these trees, we can find the most parsimoni- Fig. 2. The three possible different cladograms for the dinosaurs from Fig. 1, showing the distribu- ous (simplest) solution to the tion of derived features in each. Numbers along the branches represent the various features from distribution of features we see. Fig. 1. Note that the cladogram in 2C requires the fewest number of evolutionary changes, and so Using our example in Fig. would be the one preferred in this analysis. 2A, we find that this clado- gram is explained by 11 evo- lutionary changes: one change each for the saurus and Tyrannosaurus split off from the line three primitive features shared by all; one leading to Deinonychus, but before the ances- change each for the two unique Deinonychus tors of Albertosaurus and Tyrannosaurus split features; and the remaining two features each from each other. So, all other things being evolving independently—once each on the line equal, we would choose the cladogram in Fig. leading to Tyrannosaurus, and once each on the 2C, with the fewest number of evolutionary line leading to Albertosaurus. Similarly, the cla- changes, as representing the closest approxima- dogram in Fig. 2B shows a similar distribution tion to the actual historical pattern of common requiring 11 changes, but in this case the ancestry. changes in features 8 and 9 are explained as re- Is the cladogram in Fig. 2C the true histori- versals (evolving back from the derived state), cal pattern? We can’t know for certain. It is only having been present in the common ancestor our best hypothesis for the pattern, given the data of Tyrannosaurus, Albertosaurus, and Deinon- at hand. An advantage to the cladistic method is ychus, then reversed along the line leading just that its results can be subjected to further tests. to Deinonychus. Finally, the cladogram in Fig. For example, new taxa or new features can be 2C requires only 9 steps, with features 6 and 7 added to the analysis, and if the simplest distribu- being present in the common ancestor of tion of derived features in the new analysis Deinonychus, Albertosaurus, and Tyrannosaurus, matches the previous results, the original hypoth- and features 8 and 9 evolving only a single esis is supported. If not, the original hypothesis time—after the common ancestor of Alberto- would be rejected in light of new observations.

D inosaurs: The Science Behind the Stories 35 D inosaurs:A Time and Place

Advantages of the One method, favored by many biologists Cladistic Method (including the majority of dinosaur paleontolo- gists), is a scheme proposed by Hennig himself, That new analyses can be run with the addi- in which all taxon names represent tion of new data gives cladistic analyses distinct (complete branches of the Tree of Life). Each advantages over the traditional method of sim- named group represents an ancestor and all of ply connecting possible ancestors and possible its descendants. For example, on cladogram descendants through time. A cladistic analysis Fig. 2C, node “a” would represent the clade searches for patterns of relative common ances- Tyrannosauridae containing, in this very re- try, something we can be more secure about. duced cladogram, only Tyrannosaurus and At some level there is a common ancestor for Albertosaurus. Node “b” would be the larger every pair of species that ever existed. Neither group , containing , nor a dog, nor a dandelion is an Tyrannosauridae and Deinonychus; and finally ancestor or descendant of the others, but we node “c” would be (Allosaurus can still recognize that the first two shared a common ancestor not shared by the plant. plus the coelurosaurs). Over time, some mem- Thus, even if some, or even many, of the indi- bers within a clade might become more and vidual species along the actual branches of the more transformed from the ancestral condition Tree of Life are missing, we can approximate as new features evolved. However, they would its shape, understanding that additional data still be considered part of the clade because may change that approximation. they descended from that common ancestor. Additionally, the nature of cladistic analy- ses makes them relatively easy to conduct using So, What Are Dinosaurs? computer programs. After a scientist has coded A Basic Classification the observations of derived features for various Using the methods outlined above, the posi- taxa and entered those data into a search pro- tion of dinosaurs within the Tree of Life (Fig. gram, a computer can sort through millions, or 3) and the details of the cladogram of the dino- even billions of trees, searching for the best— saurs themselves (Fig. 4) have been recon- most parsimonious—results. So, more and structed. Although broad consensus exists more complex analyses, each with dozens of about most of the details, there are sometimes taxa and hundreds of features, can be con- discrepancies in the exact results obtained in ducted far faster than a being could different studies.7, 8, 9, 10 These discrepancies ever hope to achieve with a pad of paper and a may reflect the choice of features selected for pencil. Such studies are now commonplace comparison, or may simply be a reflection of throughout biology and , using lack of information about some aspects of the not only the shapes of , but also soft tis- in question. sues, DNA sequences, and even behavioral fea- Cladistic analyses of the tetrapods consis- tures. These studies often result in many equally tently show that dinosaurs are a subgroup simple cladograms, all of which are equally good within the archosaurian reptiles, a group that approximations given the data available. Better includes crocodilians among living forms, as approximations will be reached by running well as many extinct creatures.11 Note that call- analyses as more evidence is gathered. ing dinosaurs “reptiles” in the cladistic tells us nothing about whether dinosaurs were Cladistic Classifications cold-blooded or warm-blooded. Like all cladis- Darwin put forth the idea that a good organiz- tic names, Reptilia is a group defined by com- ing principle for classification would be pat- mon ancestry, rather than by a general grade of terns of common ancestry. Hennig’s method of organization. Dinosaurs and their closest rela- cladistic analysis allows the most likely pattern tives, such as , are distinguished of common ancestry to be recovered and used from all other reptiles by a fully upright stance as a basis of classification.6 of the hindlimbs and a simple ankle . Fur-

36 D inosaurs: The Science Behind the Stories Chapter 4—Classifying Dinosaurs

thermore, Dinosauria itself is distinguished by the plated , tank-like , an open hip socket. Because of the incomplete- and their closest relatives; the ridge-headed ness of the fossils of the immediate closest rela- , including the dome-skulled tives of dinosaurs, there is some uncertainty as and the parrot-beaked to what other features characterize the com- (most famously including Tricer- mon ancestor of all dinosaurs. atops and the other horned Two main branches exist within Dinosauria: Lagosuchus Dinosauria Fig. 3. and Saurischia. The ornithis- (Middle relative (dinosaurs) chians include and Hylaeosaurus Pterosauria of dinosaurs) A cladogram among the original three members of (flying reptiles of showing the Pseudosuchia the Mesozoic) Owen’s Dinosauria, as well (crocodilians & their Dinosauromorpha position of as all the dinosaurs Lepidosauromorpha extinct relatives) (upright hindlimbs) (, snakes & their Ornithodira dinosaurs sharing a extinct relatives) (hinged ankle) Anapsida among the ( & their Archosauria Synapsida extinct relatives) (opening before eye) four-limbed ( & their Diapsida extinct relatives) (paired openings on side of ) ); . Amphibia (modern amphibians & Reptilia and the beaked Ornitho- their extinct relatives) (extra opening in rear of skull) poda, including Iguanodon and Amniota (amniotic ) the many diverse species of duckbills. Tetrapoda The Saurischia include Megalosaurus and (four limbs with ) all the dinosaurs more closely related to it than more recent common to Iguanodon. One clade of saurischians is the ancestor with them than with Megalosaurus. At long-necked herbivorous group Sauropodo- present all known ornithischians seem to have morpha. Sauropodomorphs include primitive been herbivorous. In fact, they all had an extra bipedal forms like Saturnalia and Fig. 4. jawbone (the predentary) which held part of a as well as the gigantic quadrupedal A basic horny for chopping up plants. All but the (such as , , and cladogram of the most primitive had a backward-pointing pubic ). True sauropods are the largest relationships bone, presumably allowing for the increased animals ever to walk the land. among the major gut space necessary to digest large amounts of Perhaps the most diverse clade of dinosaurs vegetation. Major groups of ornithischians in- is the Theropoda. Theropods contain the vari- groups of dino- clude the armored Thyreophora, comprising ous carnivorous dinosaurs, from tiny Compso- saurs.

Avialae () Hadrosauridae Hypsilophodontia Therizinosauroidea Heterodontosauridae Ceratopsia Pachycephalosauria Ankylosauria Coelurosauria Stegosauria Marginocephalia Sauropoda Spinosauroidea Thyreophora Avetheropoda Saturnalia

Pisanosaurus Theropoda

Ornithischia Saurischia

Dinosauria

D inosaurs: The Science Behind the Stories 37 D inosaurs:A Time and Place

gnathus and through giants such as species of dinosaurs in each of the major Tyrannosaurus, , and Giganotosau- groups. Furthermore, by using the principles rus. Birds represent descendants of small car- of parsimony and an understanding of the nivorous dinosaurs (see Currie, page 89). Indeed, shape of the Tree of Life, paleontologists are by the principles of cladistic classification birds attempting to discover the pattern of evolu- are members of the Dinosauria. tion of various aspects of the biology and The methods of cladistic analysis continue behavior of dinosaurs. Recovering the interre- to hold great promise for our understanding of lationships among the dinosaurs and their po- the evolution of the different groups of dino- sition among the vertebrates is only the first saurs. New studies continue to determine step in exploring the evolution of this amazing more precisely the relationships of the various group of reptiles.

R eferences

1. Owen, R. 1842. Report on British fossil reptiles, Part II, 60–204. Report of the Eleventh Meeting of the British Association for the Advancement of Science for 1841. 2. Linné, C. 1758. Systema natura per regina tria naturae, secundum classes, ordines, genera, species cum characterisbus, differentiis, synonymis, locis. Editio decima, reformata, Tomus I: Regnum Animalia. Stockholm: Laurentii Salvii. 3. Darwin, C. 1859. The origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. (Complete text of the first edition is available online at www.literature.org/ authors/darwin-charles/the-origin-of-species/) 4. Hennig, W. 1950. Gründzuge einer theorie der phylogenetischen systematik. : Deutscher Zentralverlag. 5. Hennig, W. 1966. Phylogenetic . Urbana: University of Illinois Press. 6. Holtz, T.R., Jr., and M.K. Brett-Surman. 1997. The taxonomy and systematics of the dinosaurs. In The complete dinosaur, eds. J.O. Farlow and M.K. Brett-Surman, 92–106. Bloomington: Indiana University Press. 7. Sereno, P.C. 1997. The origin and evolution of dinosaurs. Annual Review of Earth and Planetary Sciences 25:435–489. 8. Sereno, P.C. 1999. The evolution of dinosaurs. Science 284:2137–2147. 9. Holtz, T.R., Jr. 2000. Classification and evolution of the dinosaur groups. In The Scientific American book of dinosaurs, ed. G.S. Paul, 140–169. New York: St. Martin’s Press. 10. Pisani, D., A.M. Yates, M. Langer, and M.J. Benton. 2002. A genus-level supertree of the Dinosauria. Proceedings of the Royal Society Series B 269:915–921. 11. Brochu, C. 2001. Progress and future directions in . Journal of Paleontology 75:1185–1201.

38 D inosaurs: The Science Behind the Stories