Early Terrestrial Animals, Evolution, and Uncertainty

Abstract

Early terrestrial ecosystems record a fascinating transition in the history of life. Animals and plants had previously lived only in the oceans, but, starting approximately 470 million years ago, began to colonize the previously barren continents. This paper provides an introduction to this period in life's history, first presenting background information, before focusing on one animal group, the arthropods. It gives examples of the organisms living in early terrestrial communities and then outlines a suite of adaptations necessary for survival in harsh terrestrial environments. Emphasis is placed on the role of uncertainty in science; this is an integral part of the scientific process, yet is often seized upon by god-of-thegaps creationist arguments. We hope to illustrate the importance of both uncertainty and scientists' freedom to express doubt while a consensus is being built.

Ancestral reconstruction of tick lineages

“…The respiratory systems of terrestrial arthropods are internal and are composed of either book lungs (considered ancestral), or internal tracheal networks that are considered to have evolved numerous times independently, even though all possess spiracles (Garwood and Edgecombe, 2011;Klok et al 2002). The general tracheal arrangement is conserved in parasitiform mites .…”

“…Arthropods utilize the Malpighian system for further prevention of water loss by concentrating waste in the form of solid uric acid and guanine and readsorbing water into the body (Garwood and Edgecombe, 2011). Waste products are secreted into the post-colon.…”

“…Malpighian secretion has been observed in soft ticks, but is generally not considered a major form of secretion (Jupp et al 1987). It does suggest that this form of secretion may have been ancestral to ticks, since it is also found in other arthropods (Garwood and Edgecombe, 2011). Conversely, coxal organs are also found in other…”

“…Acari and arthropods and function as osmoregulatory organs Garwood and Edgecombe, 2011), with the implication that these also had ancestral precedence and were lost in the Ixodidae and Nuttalliellidae. Ixodids retain an accessory coxal gland present in argasids, thought to function during ecdysis (Binnington, 1975).…”

“…However, evolutionary stasis may be expected in higher-level taxonomic groups with relatively low speciation rates and has been incorporated into a number of evolutionary models, even if these are the exception rather than the rule (Benton and Pearson, 2001;Cavalier-Smith 2006;Eldredge and Gould, 1972;Garwood and Edgecombe, 2011; Figure 4: Reconstruction of blood-feeding characteristics in the Ixodida. Indicated are the date estimates for various nodes of the Ixodida and corresponding major events in evolution.…”