Australia: The Land Where Time Began

A biography of the Australian continent 

The Devonian World – Early Evolution of Plants and Animals

The earliest record found so far of land plants are plant fragments containing spores, that are thought to be from liverworts, in rocks of the Late Ordovician (Wellman et al., 2003). Little else is known about life on land until the beginning of the Silurian. Organisms such as liverworts, lichens, fungi, and mosslike plants were present that could inhabit humid terrestrial nearshore habitats (Kenrick & Crane, 1997), and from late in this interval there is evidence of terrestrial arthropods (Jeram et al., 1990). Plants such as Cooksonia, leafless, sticklike plants that had no true roots, could grow around the shores of shallow lakes and rivers. According to Clack¹ significant vegetation would have been present only close to the edge of the water and in the shallows, and it was here that deposits of decaying plant matter may have formed sufficient organic debris to provide a footing for plants to send up shoots, which would allow invertebrates to climb on and around them. Plant communities are known from some sites (Tomescu & Rothwell, 2006), that were quite complex by the end of the Silurian.

Millipedes from the Middle Silurian were the earliest known body fossils of terrestrial arthropods which are among the earliest known air breathing animals (Wilson & Anderson, 2004). Trackways in earlier rocks were recognised as probably being made by millipedes, and suggest that some arthropods were terrestrial for at least part of the time as early as the Ordovician, and may have been air breathers (Shear & Seldon, 2001), By the end of the Silurian true vascular plants that had water-conducting tissue and pores for gas exchange had evolved. During the Devonian arthropods such as millipedes, scorpions and trigonotarbids, which were distantly related to spiders, appeared in numbers in the fossil record and were living among the plants (Rolfe, 1980; DiMichele & Hook, 1992). None of these plants grew very large, forming little cover for the animals to seek shelter in. Each plant type tended to be restricted to its own small colony, with low levels of interaction between plant types, which differs from the situation at the present. The vegetation expanded and diversified by the start of the Devonian and throughout the early part of the period, and the flora can be divided into provinces according to latitude by the Middle Devonian. The Devonian is characterised by land plant and invertebrate diversification which resulted in a rich, diverse biota by the close of the Devonian. Clack¹ suggests the increased vegetation cover appears to have been enough to change the atmospheric balance to one in which CO₂ was decreased as it was removed from the atmosphere by photosynthesising vegetation.

Sources & Further reading

1. Clack, JA, (2012). "Gaining Ground: The origin and evolution of tetrapods", Indiana University Press