Morphology, Structure, and Function of Fossils — Introduction to Paleontology (Part V)

Morphology, Structure, and Function of Fossils

When we look at living organisms today, we often become curious about their outward appearance. Why does a giraffe have such a long neck? Why does a peacock have such long and beautiful tail feathers? Why does a bee have a stinger on its abdomen for defense? With living organisms, we can directly observe and study their morphology, structure, and function. Paleontologists feel the same curiosity when they confront fossils. Some ancient organisms possess forms unlike anything seen in living organisms today, which makes us even more eager to understand what opportunities and environmental conditions once shaped such forms. The morphology preserved in fossils is often the only source of information paleontologists have for understanding those organisms, yet morphology alone can already provide a great deal of insight. If a fossil is sufficiently complete, it can reveal the taxonomic classification of the organism, individual variation, its place in an evolutionary tree, and even aspects of its behavior and former ecological role.

Individual differences within a species

If we use humans as an example, every one of us has a unique appearance, and the same is true of other organisms. Therefore, when observing fossil morphology, one must also take individual variation within a species into account so that members of the same species are not mistakenly identified as different species. Several kinds of individual variation are described below:

1. Individual diversity: except for identical twins, every individual within a species shows slight differences. These differences arise from partially different DNA sequences or from epigenetic effects. Individual diversity is also one of the important driving forces of evolution (Figure 1).

2. Geographic diversity: populations of the same species living in different places may show even greater differences, and subspecies may even arise. This is also one transitional route in the formation of new species (Figure 2).

3. Sexual dimorphism: in some species, males and females may differ greatly in appearance even though they belong to the same species, as in anglerfish, mandarin ducks, eclectus parrots, and deer (Figure 3).

4. Growth stages: in some species, juveniles and adults may look completely different. This is especially common in insects that undergo complete metamorphosis (Figure 4). Even among vertebrates, the appearance of a lamb differs from that of an adult ram with large horns.

5. Ecophenotypic variation: ecological conditions can influence the growth form of a species, a phenomenon especially common in plants. For example, Hieracium umbellatum growing on seaside cliffs develops broader leaves and larger inflorescences, whereas individuals growing on sandy ground have narrower leaves and more compact inflorescences. Among animals, individuals of the mollusk Nucella lamellosa that grow in deeper water develop shells with longer protruding spines.

（Figure 1）Sparrows show slight pattern differences between individuals（圖片來源：Hedera.baltica，採用 CC BY-SA 2.0 授權）

（Figure 2）Common grackles (Quiscalus quiscula), left near the U.S. interior, right near Florida（左，圖片來源：Heather Paul，採用 CC BY-ND 2.0 授權；右，圖片來源：Rhododendrites，採用 CC BY-SA 4.0 授權）

（Figure 3）Sexually dimorphic eclectus parrots, female on the left, male on the right（圖片來源：Shiny Things，採用 CC BY 2.0 授權）

（Figure 4）Some insects not only differ between larvae and adults but also have specialized castes（圖片來源：Tate Holbrook，採用 CC BY-SA 3.0 授權）

Allometry and isometry

As mentioned above, growth stage is also one aspect of individual variation. In some animals, juveniles and adults differ in form. Human infants, for example, have relatively larger skulls and shorter limbs, whereas in adults the skull is proportionally smaller relative to the body and the limbs are longer. Similar patterns can also be seen in ancient organisms. Juveniles of Ichthyosaurus communis, for instance, had a noticeably larger head in proportion to the body (Figure 5). This phenomenon is known as allometry, meaning that different body parts grow at different rates. If a body part becomes proportionally larger during growth, this is called positive allometry, as in antlers or limbs; if it becomes proportionally smaller, this is called negative allometry. Isometry, by contrast, refers to growth in which the whole body enlarges in equal proportion. This growth pattern is less common than allometry. One of the most familiar examples is the salamander, whose adult form is almost simply an enlarged version of the juvenile (Figure 6).

（Figure 5）Juvenile fossil of Ichthyosaurus communis（感謝Dean Lomax提供）

（Figure 6）Isometric growth in salamanders（圖片來源：Josèphe Huët，CC0 1.0 公共領域）

Functional morphology

In living animals, we can observe behavior directly in order to understand functional morphology. When we see birds flying in the sky, we know that their wings function in flight; an elephant's trunk is used not only for breathing, but also for picking up objects and helping with drinking. Understanding the functional morphology of ancient organisms is far more difficult. How fast could a trilobite crawl? What did a pterosaur look like in flight? Why did saber-toothed cats have such long canine teeth? There are three main approaches that can be used to infer the function of an organism from fossil evidence.

1. Comparison with living animals of similar form

If members of the taxonomic group to which an ancient organism belonged are still living today, it becomes easier to analyze the fossil's functional morphology. For example, Devonian lamprey fossils can be compared with modern lampreys. However, some ancient organisms within the same group may differ enormously from their modern relatives. Many theropod dinosaurs, for instance, cannot be fully understood simply by comparison with modern birds. Living birds are generally small-bodied, and many aspects of their anatomy are specialized for flight. This makes them an incomplete model for interpreting the behavior of large-bodied theropod dinosaurs. From the perspective of phylogeny, dinosaurs should be compared with both crocodilians and birds, since crocodilians are also among their closest living relatives. Crocodilians and birds each have their own evolutionary histories, and no one can guarantee that every trait they possess was also present in dinosaurs. However, if both crocodilians and birds share a certain trait, then there is a strong chance that dinosaurs also possessed it. This method is called extant phylogenetic bracketing (EPB): using two or more modern close relatives to infer the characteristics of an extinct organism from the traits they share. EPB can sometimes provide very useful information, especially about soft tissues that rarely fossilize. If modern relatives share similar soft-tissue structures, it may be possible to infer that the ancient organism had comparable soft tissues, allowing a deeper anatomical interpretation.

In some cases, however, modern close relatives do not provide the most appropriate information. Take the giant Tyrannosaurus as an example: when crocodilians or birds are used as brackets, certain traits such as movement may be distorted. In such cases, paleontologists may instead compare it with animals closer in body size, such as elephants, even though elephants are evolutionarily very distant from dinosaurs. Likewise, if an extinct member of a group was carnivorous but its living relatives are herbivorous, then comparing their tooth structure would be highly unreasonable, because carnivores and herbivores have completely different dentitions. In that case, comparison with unrelated carnivorous animals may yield a more accurate interpretation.

2. Biomechanical modeling

Increasingly, paleontologists now use biomechanical principles and modeling to infer the movements of ancient organisms. By using computers to perform mechanical calculations on the body structure of an animal, researchers can reconstruct motions and forces that best match its physical form. There are many computational methods, and results often require the integration of multiple approaches. One important method is the finite element method (FEA). This method was originally used to calculate the strength of structures such as bridges and houses, but in paleontology it is now mainly used to evaluate how force is distributed across an organism's body structure in order to infer possible movements.

If we want to know how Tyrannosaurus ran, for example, we must first analyze which bones were involved in running, reconstruct the size of the muscles attached to those bones, estimate the range of motion of the legs, evaluate the points where forces were applied, observe running ostriches, and create 3D simulations of how Tyrannosaurus could have run most plausibly without falling over. Only then can a computer animation be used to reconstruct its running posture. Even so, the final analysis may not yield a single answer, and further evidence is often needed before the most accurate biomechanical interpretation can be identified.

3. Matching clues from other related fossils

Functional morphology can also be inferred from trace fossils. Footprint fossils, for example, can reveal the distance between footprints and the shape of the steps, making it possible to estimate how an animal walked or ran and how fast it moved. Fossils showing bite marks, claw marks, or impact damage can also reveal animal behavior and the amount of force involved. In addition, fossils of other species found nearby, together with the position of the fossil-bearing layer, may indirectly reveal the ecological role of the organism and the environmental and climatic conditions in which it lived. From such information, its way of life may be inferred. Even scattered and fragmentary evidence may sometimes be pieced together into something highly informative.

Accurate reconstruction of ancient organisms is not easy. It requires the integration of many different lines of analysis in order to arrive at a result that is closest to reality. It is hoped that this introduction will help readers better understand some of the factors that must be considered in the process of reconstructing ancient life.

Author: Shui-Ye You