Endocasts

Text by Ted Macrini

Introduction

An endocast is a 3D representation of the space within a cavity. The most commonly studied endocasts of vertebrates are cranial endocasts, or 3D representations of the space within the cranial cavity (= endocranial space), which is filled in life to some degree by the brain. Other cavities of the vertebrate skull, such as the inner ear and nasal cavity, are also studied using endocasts.

Endocasts are important for reconstructing soft tissue anatomy of sensory organs, particularly in extinct animals. Soft tissue structures of vertebrates, such as organs, only fossilize under extraordinary conditions, as is the case with frozen Pleistocene mammals from northern Russia and Alaska (Farrand, 1961; Guthrie, 1990). Because of the extreme rarity of this type of soft tissue preservation, paleontologists rely heavily on cranial endocasts to study the brain and central nervous system in extinct animals. This branch of paleontology dealing with the fossil record of the nervous system is known as paleoneurology.

Cranial endocasts provide better approximations of the brains of some vertebrates than others based on the degree to which the brain fills the endocranial space (Jerison, 1973). The brains of mammals, birds, and at least some non-avian dinosaurs largely fill the endocranial space leaving an impression on the internal surfaces of skull bones. Because of this, the importance of cranial endocasts for studying the evolution of the brain in fossil members of these groups has long been recognized (e.g., Marsh, 1884; Simpson, 1927, 1937; Edinger, 1942, 1948, 1949, 1955, 1964, 1975; Radinsky, 1968a, b, 1973a, b, 1976, 1977; Hopson, 1979; Jerison, 1973, 1991; Kielan-Jaworowska, 1983, 1984, 1986; Hurlburt, 1996; Rowe, 1996a, b; Franzosa, 2004).

It is important to point out that, besides the brain, the cranial cavity houses other soft tissue structures such as the meninges, blood vessels, and nerves, and therefore cranial endocasts only provide approximations of external features of the brain. Even so, the general shapes and volumes of some external features of the brain can be inferred from endocasts. However, cranial endocasts do not provide any direct information about the internal structure of the brain such as morphology of the neurons, number of neurons, neuron density, or neuron connectivity (Deacon, 1990). These absolute data can only be obtained from the brains themselves.

The endocasts used to study endocranial space may either be naturally occurring or artificially generated. A natural endocast (Steinkern) is formed when sediment fills the cranial cavity of a skull and then lithifies. Natural endocasts are often exposed as the skull breaks and weathers away. Artificial endocasts are often made to visualize the endocranial space of skulls that lack a natural endocast. Artificial endocasts can be generated using conventional techniques by, for example, constructing a latex internal mold of the endocranial cavity, extracting the mold through the foramen magnum, and then using the mold to make a plaster cast (e.g., Radinsky, 1968a).

For many fossil specimens, it is not possible to generate artificial endocasts from latex molds because the cranial cavity is filled with matrix. In the past, the endocranial space of these skulls was studied using destructive techniques. This usually involved either serial sectioning of the skull (e.g., Sollas, 1904) or physical removal of surrounding bones of the braincase to reveal the natural endocast (e.g., Hofer and Wilson, 1967). These methods are obviously unfavorable for studying rare or unique specimens.

An alternative, non-destructive approach to studying these specimens is to generate digital artificial endocasts. Digital artificial endocasts are generated by first digitizing a skull via magnetic resonance imaging (MRI) or computed tomography (CT), and then digitally isolating the space within the endocranial cavity. Digitally generated endocasts are often referred to as ‘virtual endocasts.’

Why Study Endocasts?

Comparisons of the relative sizes of gross structures of the brains of extant animals are used to infer the degree of evolution of different sensory systems associated with the brain (Jerison, 1973; Butler and Hodos, 1996). This is based on the ‘principle of proper mass’ which states that the mass of the neural tissue of a particular segment of the brain is correlated with the amount of information processing involved in performing that particular function (Jerison, 1973:8). A related assumption, based on observations on extant mammals, is that gross structures of cranial endocasts of mammals provide reasonable proxies for the size of the corresponding brain feature (Edinger, 1948; Jerison, 1973). Therefore, comparative studies of different portions of endocasts of extinct mammals provide information about the evolution of different sensory systems (e.g., Radinsky, 1968a, b, 1973a, b, 1976, 1977). For example, an endocast with relatively large superior colliculus casts suggests that this individual and presumably its species had large superior colliculi and more acute eyesight in comparison to an endocast with smaller superior colliculus casts. Studies based on these types of comparisons between regions of the cranial cavity are crude, but endocasts are the best available information about the central nervous system and sensory systems of extinct taxa.

Study of the sensory systems of organisms is important for understanding behavior of those organisms. Behavior is response to stimuli and the brain is the organ in which sensory information and motor functions are coordinated. The evolution of behavior is tied to the evolution of the brain, and therefore cranial endocasts are useful for studying the behavior of extinct animals.

In addition, cranial endocasts represent a potentially large amount of unexplored phylogenetic data. The majority of morphological data for phylogenetic analyses of vertebrates comes from the exterior of the skull, including the dentition. Internal cranial morphology is poorly represented in phylogenetic analyses because of the difficulty in visualizing and studying this anatomy. The advent of CT technology has revolutionized the collection of data on the internal cranial morphology of vertebrate skulls (e.g., Rowe et al., 1995, 1999, 2005; Brochu, 2000; Larsson et al., 2000; Tykoski et al., 2002; Witmer et al., 2003; Maisey, 2004, 2005; Van Valkenburgh et al., 2004; Colbert et al., 2005; Franzosa and Rowe, 2005), and therefore provides the potential to incorporate these new data into phylogenetic analyses.

New Approaches to Previous Endocast Studies

The lack of natural cranial endocast material for a number of fossil vertebrates is an impediment to endocast studies. Furthermore, it is impossible to non-destructively generate artificial endocasts from many fossil skulls using conventional techniques. CT technology has been successfully employed to digitize skulls for non-destructive extraction of digital endocasts from fossil skulls (e.g., Brochu, 2000; Larsson et al., 2000; Marino et al., 2000, 2003; Witmer et al., 2003; Franzosa, 2004; Franzosa and Rowe, 2005; Macrini et al., 2006).

Many of the natural cranial endocast specimens that are available for study are incomplete or cannot be studied in all views (e.g., the basicranial bones often obscure the ventral surface of endocasts). Because of this, it is difficult to take accurate linear and volumetric measurements from natural endocast material. CT technology can remedy this problem. Volume measurements from CT data in some cases are more accurate than volumes taken from natural endocast material (Macrini, 2006).

Endocasts on DigiMorph

Virtual endocasts are available on DigiMorph under the 'Additional Imagery' tab for the following taxa:

Non-vertebrates:
Quercus robur (pedunculate oak)

Lizards:
Shinisaurus crocodilurus (Chinese crocodile lizard)

Pterosaurs:
Anhanguera santanae
Rhamphorhynchus muensteri

Dinosaurs including Birds:
Acrocanthosaurus atokensis (theropod dinosaur^†)
Alioramus altai (tyrannosauroid dinosaur^†)
Anas platyrhynchos (domestic duck)
Apatosaurus sp. (sauropod dinosaur^†)
Archaeopteryx lithographica (fossil avialan)
Bucorvus abyssinicus (northern ground hornbill)
Incisivosaurus gauthieri (oviraptorid dinosaur^†)
Phoenicopterus ruber (Caribbean flamingo)
Saurornithoides mongoliensis (theropod dinosaur^†)
Zanabazar junior (theropod dinosaur^†)

Mammals and their ancestors:
Bathygenys reevesi (oreodont^†)
Canis lupus (gray wolf)
Dasypus novemcinctus (nine-banded armadillo)
Dasyurus hallucatus (northern quoll)
Didelphis virginiana (Virginia opossum)
Felis sylvestris (feral domestic cat)
Hadrocodium wui (mammaliaform^†)
Herpetotherium fugax (metatherian^†)
Manis tricuspis (African tree pangolin)
Monodelphis domestica (gray short-tailed opossum)
Morganucodon oehleri (mammaliaform^†)
Obdurodon dicksoni (fossil platypus)
Ornithorhynchus anatinus (duckbill platypus)
Orycteropus afer (aardvark)
Phascolarctos cinereus (koala)
Procavia capensis (rock hyrax)
Pucadelphys andinus (metatherian^†)
Tachyglossus aculeatus (short-nosed echidna)
Tapirus indicus (Asian tapir)
Tapirus terrestris (lowland tapir)
Trichechus senegalensis (West African manatee)
Tursiops truncatus (bottlenose dolphin)
Vincelestes neuquenianus (therian^†)
Vombatus ursinus (common wombat)
Zaglossus bruijni (long-nosed echidna)

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