The Advantages and Disadvantages of the Fossil Record in Comparison to Extant Species in Understanding Evolutionary Ecology

Ecological diversity comprises two components: the number of species present and their relative abundance within ecosystems (Raffaelli, 2007). The fossil record, however, is used to estimate the number of ancient species and their abundance in the distant past. It is, therefore, logical to study the behaviour, morphology, general biology and ecology of any present (extant) species, which are more closely related to ancient fossils. This can then help elucidate the ecological diversity of species from the past and their evolutionary ecology, i.e. how they interacted with each other and their evolutionary histories. This includes evolutionary life cycles, behaviour, inter-specific relations and the evolution of biodiversity (Fox et al., 2001; Mayhew 2006; Pianka 2000).
Fossils can also, therefore, provide insights on the ecology and behaviour of extant species, how they evolved and adapted to changing biotic and abiotic factors. Invertebrate fossils are far more numerous than vertebrate ones. This is partly due to their tough exoskeletons and decay-resistant shells (Taylor and Lewis, 2005), not just their size and reproductive capabilities. Fossilised arthropod species, especially those of arachnids, are often found beautifully preserved in amber. This form of fossil not only gives valuable information about the species but also ancient forest ecosystems. However, most arthropod species in amber are often described from singleton finds or are too few in number. This means species’ numerical abundance data are very limited (Penny and Langan, 2006). Usually terrestrial invertebrates caught in amber correlate well with underlying species’ distribution and diversity patterns of extant tropical insects at the family level (Labandeira 2005). Many Cenozoic spiders have been preserved this way and belong to extant species (Penny et al., 2003). These examples often reveal similar ecologies and behaviour to their present close relatives at the family level also (Penny and Langan, 2006).
However, it is only the Baltic and Dominican Republic ambers which have sufficient numbers of fossilised taxa for quantitative studies (Penny, 2005). Baltic amber is twice as old as Dominican amber and therefore has greater numbers of extinct taxa. The climate was very different to that in the region today, unlike Dominican amber which was formed in climates very similar to current tropical climates. The preserved Dominican amber species are directly related, similarly diverse, and thus, comparable to present spider ecologies (Penny, 2005). Despite this, fossil species richness estimates were exaggerated due to the large number of singletons (Penny, 2005).
An ecological study by Penny and Langan (2006) on size distributions of spiders trapped in Baltic and Dominican ambers revealed that aerial web-spinning spiders were larger in the Baltic. They inferred that the Baltic amber came from an unknown, possibly extinct pinaceous tree, but comparable to Pseudolarix, an extant deciduous conifer relative. This tree’s overall morphology is structurally far more complex in comparison to the amber from the Dominican leguminous tree (Penny and Langan 2006). Halaj et al. (2000) found that needle density and branching complexity supported arboreal web-spinning spiders of larger size. This could account for the increased size of related spiders in Baltic amber and not differences in the physical entrapment process between these ambers (Penny and Langan 2006).
A new species of spider belonging to the Pisauridae family was found in Burmese amber, which increases the range of this family beyond the previous discovery in Baltic amber by 60 million years (My) (Penny 2004). At 100-107 Mya, this Cretaceous species, Palaeohygropoda myanmarensis, is the oldest pisaurid spider. It infers the presence of extant spider families: Zorocratidae, Tengellidae, Amaurobiidae and others, around the same period (Penny 2004). However, there is no direct evidence to support the latter conclusion from this study. It also implies that there were freshwater habitats in this amber forest, where this is the oldest record of a spider adapted for walking across water surface films. Palaeohygropoda is closely related to the extant genus, Hygropoda, which may now require taxonomic revision (Penny 2004).
Another informative find was that of fossilised orb-web spiders, from the extant Araneidae family, preserved in Lower Cretaceous amber from Alava, Spain (Penny and Ortuño, 2006). This demonstrated that these fossilised spiders may have a common ancestor from the Jurassic period. They then evolved, diversified and co-radiated along with the Cretaceous explosion of angiosperms and their flying insect pollinators, which became these spiders’ prey (Penny and Ortuño, 2006). A very rare discovery of a Spanish, Early Cretaceous spider web (c.110 million years old) with insect, trapped in amber, supports this theory. It is the oldest direct evidence of a spider web used for prey capture from the Araneoidea (Peñalver et al., 2006).
One of the most studied and best examples of vertebrate fossil records, lies with the group of animals collectively known as proboscideans, which gave rise to today’s elephants. This is partly due to better preservation of large animal bones along with the greater likelihood of discovering them (Sukamar, 2003). The fossil record provides a fascinating evolutionary and ecological history with respect to changing size, morphology and diversification. The extant Asian elephant (Elephas maximus) and African elephant (Loxodonta africana) are living representatives of more than 160 past proboscidean species (Sukamar, 2003). Until about 10,000 years ago many of these species still wandered the globe, before rapid extinctions. Predatory human species and rapid climate change were thought to be major factors (Sukamar, 2003).
During the late Eocene the climate grew cooler and drier, thus reducing lush tropical vegetation and encouraging growth of tougher, drier vegetation of lower quality. This meant proboscideans had to consume more plant material to gain enough nutritional value, which may explain increased size (Sukamar, 2003). Deinotherium giganteum and Mammuthus meridionalis grew to heights of 4 m, much larger than today’s largest African elephant. Conversely, a rapid shrinkage in size produced the dwarf elephants, such as Elephas falconeri, from Mediterranean islands such as Malta, during the Pleistocene. The Woolly Mammoth (Mammuthus primigenius) shrank by 40% over 5000 years on Wrangel Island, Siberia (Sukamar, 2003)!
The usefulness of extant elephant physiology in predicting body mass from certain fossil bones of proboscideans was demonstrated by Christiansen (2004). It was found that length and least circumference of long bone parameters were the best parameters for the prediction of body mass when comparing extant elephant species with fossil proboscideans. Christiansen (2004) also calculated that the basal and field metabolic rates of extant elephants are lower than expected for a hypothetical animal, when considering their body mass and survival on low quality vegetation. The quantities of food which was thought to be consumed by extant elephants would have sustained much larger species (Christiansen 2004).
Proboscidean fossil shows changing skull and tooth shape through time. They evolved more abrasive teeth with high crowns, increased cusp rows and thicker enamel to cope with the increased wear of tougher plant material. Another biotic factor in this change was the evolution of grasses, where moderate herbivory actually stimulated grass productivity. This new resource had to be exploited to maintain evolutionary advantage along with other competing herbivorous mammals (Sukamar 2003).
Modern molecular systematics sometimes disagree with traditional (morphological and palaeontological) classification and its conclusions. Molecular data derived from Asian and African elephants have clarified the classification of geographically isolated species thought to be different sub-species. The Sri Lankan elephant, Elephas maximus maximus, and the mainland Indian elephant Elephas maximus indicus should not be separated, based on mitochondrial haplotype studies (Sukamar 2003). Tangley (1997) suggested that the African forest elephant, Loxodonta africana cyclotis, might be a separate species from the African savanna elephant Loxodonta africana africana. Further molecular genetic comparisons with larger elephant samples, combined with morphological data, and would clarify these investigations. This highlights the problems assigning distinct taxonomic divisions to extant species let alone extinct species.
Taplin and Gregg (1989) re-interpreted Eusuchian crocodile zoogeography in light of recent evidence from systematic relationships, the ability to migrate and disperse widely in marine environments and fossil history. They maintained that physiological adaptations to a marine environment had a major influence on eusuchian natural history. The extant freshwater Gavialis and Tomistoma, may have had a salt-water ancestry. The buccal morphology of both genera suggests this, along with lingual gland similarities in Tomistoma and true crocodiles, which pre-dated the divergence of tomistomine and crocodyline stocks. The fossil record did not contradict this hypothesis (Taplin and Greg 1989).
The Mesozoic and Cenozoic produced many crocodylian fossils and, though most were conservative in body shape, some produced quite different morphologies. The most ancient forms were the terrestrial sphenosuchians which had long limbs. They were probably capable of walking on two hind legs as well as on all fours – a characteristic inherited from their common archosaurian ancestors (Elgin 2004). Archosauria were a diapsid group which included dinosaurs, pterosaurs, ‘thecodontians’ and crocodiles. They were defined mainly by having an antorbital fenestra (Glut 1997), which has been lost in modern crocodylians (Wikipedia 2007). The genus Euparkaria, from the early Triassic, is representative of the time when the archosaurian group split in two: Crocodylotarsi and Ornithosuchia (Glut 1997). The former groups lead to crocodiles, the latter to dinosaurs and birds.
Marine crocodylians evolved from the Early to Late Jurassic, such as Teleosaurus which had a very long, slender snout, like extant gharials, indicating they were also adapted for preying on fish (Elgin 2004). The extant freshwater Crocodylus johnstoni, C. intermedius and C. cataphractus all possess long narrow snouts. However, the first two fish-eaters are known to prey on anything they can tackle, including small mammals and arthropods (Brochu 2002). It is, therefore, not possible to establish with certainty, the feeding habits of ancient fossil species based solely on morphological characteristics. Extant, related species are a valuable supplement but not definitive proof of the ecology and behaviour of similar fossil species.
Fossils in the Late Cretaceous are related to the three major extant crocodilian lineages: Gavialoidea, Alligatotidea, Crocodyloidea (Brochu, 2003). Further molecular analysis has attempted to reconcile the main crocodilian phylogenetic difficulties (Brochu, 2003; Densmore and Owen 1989). Crocodylian morphological uniformity, convergence and parallelism present problems in traditional studies of the fossil relationships within this group (Brochu, 2003; Densmore and Owen 1989). Their body form has changed little for the 230 My of their life history (Elgin, 2004). Molecular studies on protein divergence, as well as mitochondrial and ribosomal DNAs, confirm the traditional conclusions linking alligators and caimans. However, they indicate that true and false gharials are more closely related than to other crocodylians (Densmore and Owen, 1989).
There is a need to be cautious about assuming molecular systematics is the main source of factual evolutionary history. During vertebrate evolution there have been three major occurrences of genome duplication which have coincided with acquisitions of physical characteristics (Donoghue and Purnell, 2005). These genetic and physical changes were thought to be linked. One duplication was thought to have given rise to jawed vertebrates (gnathostomes) (Zimmer 2006). However, studying the fossil record reveals that true gnathostome evolution was a gradual modification of body plans, not correlated with sudden leaps of complexity due to genome duplication (Donoghue and Purnell, 2005; Zimmer 2006).
The fossil record is, nevertheless, limited, often representing only one of a hundred or thousand related species and offers few characteristics (Gull 2007). For example, the amphibian and reptile fossil record in Mexico between 1869 and 2004 is poor. The fossils are often fragmentary, making their taxonomic identity difficult (Reynoso, 2006). It is not always possible to distinguish parts of fossilised bones from parts belonging to another species. Large gaps in evolutionary history become obvious when studying the proportion of extant families with a fossil record (Forey et al., 2004). The bias towards preservation of animals with tough skeletons or plants with woody bark leaves many of those soft-bodied and/or rapidly decaying species out of the record. It is only unusual environmental conditions that preserve these kinds of organisms. Recently, numerous well-preserved fossils were discovered in a Bahamian sink-hole which lacked oxygen, preventing microbial decomposition (ScienceDaily, 2007).
Morphological, taxonomic hierarchy and behavioural information from fossil species can be questioned. However, the combined study of extant species, molecular systematics and further fossil discoveries must enhance the facts about evolutionary and ecological diversity.