Line	Conodonts, the spiky phosphatic remains (bones
	and teeth composed of calcium phosphate) of
	tiny marine animals that probably appeared about
	520 million years ago, were once among the most
(5)	controversial of fossils. Both the nature of the
	organism to which the remains belonged and the
	function of the remains were unknown. However,
	since the 1981 discovery of fossils preserving not
	just the phosphatic elements but also other remains
(10)	of the tiny soft-bodied animals (also called conodonts)
	that bore them, scientists’ reconstructions of the
	animals’ anatomy have had important implications
	for hypotheses concerning the development of the
	vertebrate skeleton.
(15)	The vertebrate skeleton had traditionally been
	regarded as a defensive development, champions of
	this view postulating that it was only with the much
	later evolution of jaws that vertebrates became
	predators. The first vertebrates, which were soft-
(20)	bodied, would have been easy prey for numerous
	invertebrate carnivores, especially if these early
	vertebrates were sedentary suspension feeders.
	Thus, traditionalists argued, these animals developed
	coverings of bony scales or plates, and teeth were
(25)	secondary features, adapted from the protective
	bony scales. Indeed, external skeletons of this
	type are common among the well-known fossils of
	ostracoderms, jawless vertebrates that existed from
	approximately 500 to 400 million years ago.
(30)	However, other paleontologists argued that many of
	the definitive characteristics of vertebrates, such as
	paired eyes and muscular and skeletal adaptations
	for active life, would not have evolved unless the
	first vertebrates were predatory. Teeth were more
(35)	primitive than external armor according to this view,
	and the earliest vertebrates were predators.
	The stiffening notochord along the back of the
	body, V-shaped muscle blocks along the sides,
	and posterior tail fins help to identify conodonts as
(40)	among the most primitive of vertebrates. The lack of
	any mineralized structures apart from the elements
	in the mouth indicates that conodonts were more
	primitive than the armored jawless fishes such as the
	ostracoderms. It now appears that the hard parts that
(45)	first evolved in the mouth of an animal improved its
	efficiency as a predator, and that aggression rather
	than protection was the driving force behind the origin
	of the vertebrate skeleton.

    Even more than mountainside slides of mud or snow, naturally occurring forest fires promote the survival of aspen trees. Aspens' need for fire may seem illogical since aspens are particularly vulnerable to fires; whereas the bark of most trees consists of dead cells, the aspen's bark is a living, functioning tissue that—along with the rest of the tree—succumbs quickly to fire.

    The explanation is that each aspen, while appearing to exist separately as a single tree, is in fact only the stem or shoot of a far larger organism. A group of thousands of aspens can actually constitute a single organism, called a clone, that shares an interconnected root system and a unique set of genes. Thus, when one aspen—a single stem—dies, the entire clone is affected. While alive, a stem sends hormones into the root system to suppress formation of further stems. But when the stem dies, its hormone signal also ceases. If a clone loses many stems simultaneously, the resulting hormonal imbalance triggers a huge increase in new, rapidly growing shoots that can outnumber the ones destroyed. An aspen grove needs to experience fire or some other disturbance regularly, or it will fail to regenerate and spread. Instead, coniferous trees will invade the aspen grove's borders and increasingly block out sunlight needed by the aspens.