Sauropod dinosaurs - a natural history
by P. Martin Sander

Sauropods were the largest animals to ever inhabit the land, culminating in truly gigantic forms in at least three lineages (Upchurch 1998, Wilson 2002). These giants are unique in exceeding the body mass of any other large terrestrial tetrapods (i.e. large mammals and other dinosaurs) by an order of magnitude (Burness et al. 2001). 

The diversity and range peak of sauropods is in the Late Jurassic with the famous faunas of the North American Morrison Formation and the African Tendaguru Beds. Although less common on the northern continents during the Cretaceous, sauropods remained diverse and widespread in Gondwana and survived until the very end of this period (about 65 Mio. years ago).

Sauropods thus saw dramatic changes in world geography (break-up of Pangea) and world ecosystems. The latter not only included the changing climates of the Jurassic and Cretaceous (including the Cretaceous hothouse) but also the major ecosystem turnover resulting from the rise of the angiosperms. While from their Late Triassic origin to the Early Cretaceous, sauropods lived in and subsisted on a gymnosperm-dominated environment (with conifers, ferns, cycads, gingkos etc.), Late Cretaceous sauropods inhabited angiosperm-dominated environments that may not have looked much different from a modern subtropical forest.

The sauropod body plan

The sauropod body plan is unique among terrestrial tetrapods: A body similar to proboscideans (elephants) among mammals is combined with a very small head on a very long neck and a long tail similar to that seen in other dinosaurs (Figure 1). With the exception of brachiosaurid and camarasaurid sauropods, the long neck appears to have been held horizontally (Seymour & Lillywhite 2000, Martin 1987, Christian 2002), with the long tail acting as a counterbalance. Brachiosaurids and camarasaurids apparently held the neck in an erect pose like a giraffe, the tail being shorter in accordance.

Figure 1: The sauropod body plan and body size. The reconstruction of Brachiosaurus is based on the mounted skeleton in the Natural History Museum of the Humboldt University in Berlin. Sauroposeidon, one of the recently described truly gigantic sauropods, is only known from a string of neck vertebrae. Based on these, the animal can be estimated to have been about 30 % larger than the Berlin Brachiosaurus. From Wedel et al. (2000).

Sauropod body size

Large body size evolved very early on and remained a hallmark throughout sauropod evolution (Dodson 1990, Sander 1999a, Upchurch 2004 in "The Dinosauria"). The discrepancy in body size between other dinosaurs and sauropods, as well as between the largest land mammals and sauropods (Figure 2), has recently been highlighted by the availability of more accurate mass estimates calculated from volume estimates based on photogrammetric measurements of actual skeletons (Gunga et al., 1999) or on scientific reconstructions (e.g. Seebacher 2001)(see also project B2 and C4). These estimates place common sauropods consistently in the 15 to 50 t category (e.g. Diplodocus 10 - 20 t, Apatosaurus, 20 - 35 t, and Brachiosaurus 30 - 50 t (Seebacher, 2001, see also Sander 1999a). In addition, there are a number of very large sauropods, e.g. the brachiosaurids Sauroposeidon (Wedel et al. 2000) and the titanosaur Argentinosaurus that are estimated to have attained a body mass of 80 - 100 t. Small sauropod species with an adult body mass below 4 - 5 t are almost unknown.

The largest representatives of other dinosaur lineages, despite generally being perceived as very big, rarely exceeded the 10 t-threshold and thus actually are in the size range of very large terrestrial mammals such as extant and fossil elephants and the fossil indricotheres (Fortelius & Kappelman, 1993). Among animals, only whales grow to a larger body mass than sauropods, but a direct comparison between these two groups is difficult because of the fundamentally different constraints of the aquatic versus the terrestrial environment.

Figure 2: Body masses of sauropod dinosaurs compared to that of mammals and other dinosaurs. Data are from the literature. For mammals, only a representative sample was used.

Evolutionary origins and more general relationships

A group termed the prosauropod dinosaurs are generally hypothesized as the closest relatives of sauropods, and both groups are united in the Sauropodomorpha. Prosauropod dinosaurs existed from the Late Triassic to the Early Jurassic and included such well-known forms as Plateosaurus from the Keuper Beds of Central Europe. Sauropods probably descended from certain prosauropods, necessitating the study of the prosauropod condition in many projects involving sauropods (see project C6 ).

Recent finds (Buffetaut et al. 2000, Buffetaut et al. 2002) pushed the time of sauropod origins back into the Late Triassic (about 210 Mio years ago). These finds also document the very rapid evolution (within a few million years after their origin) of very large body size in sauropods (Buffetaut et al. 2002, Sander et al. 2004, see also project A1). Sauropods are apparently unique among dinosaurs in this respect because all other major dinosaur lineages (Theropoda, Thyreophora, Ornithopoda, Marginocephalia) show a gradual size increase over tens of millions of years.

A short review of archosaur and dinosaur systematics may be helpful here for those readers with a non-paleontological background: Sauropodomorpha share a common ancestry with all carnivorous dinosaurs, the Theropoda (which gave rise to the birds). Sauropodomorpha and Theropoda are united in the Saurischia which, in turn, is one of the two major groups of dinosaurs, the other being the Ornithischia. Dinosaurs share a common ancestry with crocodiles, various crocodile relatives of the Triassic, and pterosaurs, the extinct flying reptiles. All of these taxa are part of a major group, called the Archosauria, that arose in the Permian, and throughout its history was characterized by a continuous improvement in locomotor performance and metabolic rate, similar to but independent of the lineage (called Synapsida)that led to today’s mammals.                   

Relationships among sauropods

In the last few years, two workers, Paul Upchurch and Jeffrey Wilson, have greatly improved our understanding of sauropod relationships, evolutionary history and biogeography by conducting in-depth phylogenetic analyses, including most sauropods (Upchurch 1995, 1998, Wilson & Sereno 1998, Wilson 2002, Upchurch 2004). The results of both analyses agree largely, the major difference being the placement of some Chinese long-necked taxa. The basalmost sauropods that are known from sufficient material are Vulcanodon and Barapasaurus from the Lower Jurassic, followed by Kotasaurus from India. Of great importance for understanding sauropod evolution are several basal sauropod genera from the Lower and Middle Jurassic of China and the Middle Jurassic of Argentina, but most of these have not been described and analyzed adequately (see project C4). Also in a somewhat basal position are the cetiosaurids from the Middle Jurassic of England. All taxa more derived than these belong to a monophyletic group called Neosauropoda which consists of the predominantly Late Jurassic diplodocids and dicraeosaurids and the Late Jurassic and Early Cretaceous brachiosaurids and camarasaurids, which in turn are most closely related to the most derived sauropods, the exclusively Cretaceous titanosaurids. 

Sauropod biology

Apart from being extinct, the challenge in understanding the biology of sauropod dinosaurs lies in their sheer size because no other group before or after had pushed the upper limits of the tetrapod bauplan to the same extent (Dodson 1990). Any comparison with living tetrapods is problematic because most sauropods had a mass that was one magnitude greater than the largest living tetrapods, the proboscideans, let alone the largest living reptiles.

Analytically, sauropod biology is best approached by dividing it up into different fields of inquiry such as life history, biomechanics, and physiology. Reviews of our current knowledge were recently provided by Farlow (1989), Coombs (1990), Dodson (1990), and Sander (1999a). Numerous individual aspects were discussed in Currie & Padian (1997)  Farlow & Brett-Surman (1997) and Weishapel et al 2004 "The Dinosauria". For the sake of brevity, we review here the salient features of sauropod biology without citing specific sources.

Life history: evidence from fossil eggs and bone histology

Sauropod dinosaurs reproduced by egg-laying. In fact, fossilized eggs of titanosaurs are rather common in Upper Cretaceous localities around the world, and provide some understanding of breeding biology (see project A2). Eggs are commonly concentrated in small clutches which in turn form entire nesting grounds. The eggs are rather small compared to the adult, and after hatching, the young were largely left to themselves. Apparently each female thus produced numerous but very small offspring.

Bone histology indicates rapid growth of the juveniles to subadult size at rates comparable to those of large mammals. Sexual maturity apparently was reached towards the beginning of the second decade of life at well below maximum size. Individual life spans seem to have been three to several decades. Subadult and adult sauropods may have lived in herds as suggested by footprint evidence. Life history parameters remain poorly quantified (see project A1).

Biomechanics: stance, locomotion, and mode of feeding

The largest sauropods clearly approached the theoretical maximum body size of a terrestrial animal which is dictated by biomechanical constraints. The four pillar-like legs of sauropod probably allowed them to progress at a moderate but not sluggish pace as attested to by their commonly preserved footprints.

Neck position has been the greatest area of controversy. Most sauropods held their neck out horizontally and probably were unable to lift it very much, suggesting that they were restricted to a medium-high browse. Brachiosaurids evolved more of a giraffe-like body type with an erect neck and longer forelimbs than hindlimbs, and obviously were high browsers. However, "normal" sauropods may also have been able to reach high forage by rearing up on the hindlimbs. Biomechanics suggests that this was possible but there is no direct evidence.

While the small size of the head is a biomechanic necessity due to its position on a long lever arm, there is considerable variation in skull structure and dentition. Some dentitions apparently served only for stripping leaves and needles off branches, while in others the more massive teeth show extensive wear indicative of some kind of chewing of the food.  The small size of the head seemingly limits food intake and communition, running counter to the expected great food requirements in these animals.

Physiology

Sauropod physiology has proven most controversial and difficult to assess. While the high growth rates indicated by bone histology argue for a greatly elevated metabolic rate, models of thermoregulation suggest, to the contrary, that endothermy driven by a high metabolic rate was unlikely. The apparent problem is heat dissipation (not retention) due to the extremely low surface to volume ratio.

Obviously, much research remains to be done because almost nothing is known about the energy budget of a living sauropod such as the potential energy intake in form of plant matter, the efficiency with which this plant matter is digested, and the cost of locomotion, respiration, and reproduction.  

Gigantism as an evolutionary phenomenon

We will now turn to gigantism as an evolutionary phenomenon. The review of this topic will be somewhat preliminary because it is the focus of the second funding period of the Research Unit.

Body size is of great importance in many fields of organismic biology, and there is always the question of what the limits of and limitations to body size are, and also what drives evolutionary body size increase, the end result of which is gigantism. Thus, if one is interested in the maximum size of a land-living animal, sauropod dinosaurs are the group to study. This is true despite the fact that sauropods cannot be studied as living animals today, and many of the results obtained by paleobiological research of the kind conducted here have a much greater margin of error than results for living animals. Because of the size difference involved between sauropods and the largest land mammals, such results nevertheless remain highly significant.

Body size tends to gradually increase in many evolutionary lineages. This is particularly well documented in vertebrates and also labeled "Cope´s rule" (see Alroy 1998). Obviously, increase in body size in a lineage has to be the result of increased fitness of the individuals of a species. Several reasons, some of these no doubt acting together, are cited in the literature:

• predation pressure: larger prey animals are less threatened by predators

• intraspecific sexual selection: in competition for females, the bigger males win (This may only result in a strong size difference between the sexes, however.)

• intraspecific competition: larger females have more or larger offspring

• intraspecific competition: larger individuals have more efficient digestion

• intraspecific competition: larger individuals may have better survival chances during adverse conditions

• interspecific competition: larger species have better access to food resources.

However, we also need to ask what limits body size. Several types of constraints can be distinguished:

• theoretical bauplan limits for all land animals (strength of materials)

• limitations of locomotor mechanics

• specific limits of certain bauplans (e.g. respiration by tracheas apparently limits insect body size)

• ecological limits        

Sauropod gigantism

Of particular interest to the problem of sauropod gigantism are ecological limits. Body size today is restricted by land mass size (Colinvaux 1978, Burness et al. 2001) because each individual of a species requires a certain fraction of the land area as food resource. If body size gets too large, population density of the species drops below the critical value for survival of the species. Remarkably, sauropod and theropod dinosaurs are more than an order of magnitude too large for the respective landmass they were inhabiting (Burness et al 2001), and somehow were able to circumvent these ecological limits. This “somehow” will clearly be a focus of the second funding period of the Research Unit.

The occurrence of typical sauropod bone histology in the oldest known sauropod (Sander et al. 2004) clearly indicates that the evolutionary process of acceleration (increase in growth rate) produced the large body size of sauropods and that acceleration was very fast. This suggests a key innovation that enabled sauropods to reach gigantic size and that was present from the beginning of the lineage. Such key innovations may have been:

•  crossing of a size threshold resulting in very low mass-specific metabolic rate (Alexander 1989)

•  improved respiration due to bird-like lung (Sander et al. 2004, see also Perry & Reuter 1999, Wedell 2003)

•  lowered cost of feeding due to large "feeding envelope" provided by long neck (feeding envelope refers to the        volume covered by moving the skull on the long neck without moving the body; Martin 1987, Stevens & Parrish 1999)

•  improved digestion through prolonged retention of the ingesta (Farlow 1987)

•  lowered cost of locomotion (Christian et al. 1999)

•  greater reproductive potential due to ovipary (Janis & Carrano 1994)

Alternatively, external physical factors such as a higher oxygen content in the Earth‘s atmosphere (Pele hypothesis; Hengst et al. 1996) or higher plant productivity due to a raised CO₂ level (Burness 2001) have been proposed to explain sauropod gigantism. We consider hypotheses involving external factors unlikely because there is no positive evidence for permanently changed physical factors for the entire time and space populated by sauropods (e.g. for a raised O₂ content of the atmosphere from the Late Triassic to the end of the Cretaceous). There is also no positive evidence for the wholesale response of the biota and ecosystems predicted by these hypotheses (e.g. there is no gigantism in other lines of terrestrial tetrapods, with the exception of theropod dinosaurs).           

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