What are conodonts?

What conodonts were remained a mystery for many years. These microfossils were variously thought to belong to annelid worms, arthropods, molluscs, chaetognaths (marine worms), fish (as teeth), and even plants. The discovery of an articulated 'conodont animal' was a significant breakthrough.

Conodonts are a group of extinct microfossils known from the Late Cambrian (approximately 500 million years ago) to the Late Triassic (about 200 million years ago). They are the only known hard parts of an extinct group of animals believed to be distantly related to the living hagfish.

General morphology

Conodonts are generally preserved as minute discrete, often denticulate (having tooth-like projections) elements of 0.3 mm to 3 mm in length, with a shape varying from coniform (tooth-like) to ramiform (bars) to pectiniform (plates). Each species is reconstructed as having a certain number of elements with various shapes, including anteriorly-positioned M and S (Sa, Sb, Sc and Sd), and posteriorly-positioned P (Pa and Pb) elements. They formed a bilaterally symmetrical apparatus located in the head region of a 'conodont animal'.

Composition and preservation

Conodonts are composed of calcium phosphate, similar to the bones and teeth of vertebrates, with a preserved colour varying from translucent and colourless through light brown to black and opaque. They are preserved in most types of marine sedimentary rocks including carbonates, shales, siltstones and cherts, and are sometimes found accumulated in vast numbers. Since their mineral composition resists dissolution, conodonts can be extracted from carbonate rock samples using a weak acid (e.g. 10% acetic acid) leaching process in the laboratory.

Classification

What conodonts were remained a mystery for many years. They were variously thought to belong to annelid worms, arthropods, molluscs, chaetognaths (marine worms), fish (as teeth), and even plants. The discovery of an articulated 'conodont animal' in the Carboniferous rocks of Scotland and in the Ordovician of South Africa supports their current classification as an extinct group (Class Conodonta) of craniates, which includes jawless fish through to jawed vertebrates in the Phylum Chordata.

Function and anatomy

Based on the discoveries of bedding plane assemblages, fused clusters and, more importantly, complete assemblages found in the 'head' region of 'conodont animals', conodonts are interpreted as feeding apparatuses. This interpretation is also supported by recent studies of conodont histology, microstructure and surface micro-wear.

Ecology

With a fossil history of some 300 million years (twice as long as that of dinosaurs), conodont animals appear to have been one of the highly successful groups that lived in the ancient oceans. They lived in a wide range of habitats, from shallow inter-tidal regions to the deep sea and from warm tropical zones to cold high latitudes. These small, worm-like creatures of a few to tens of centimetres in length probably had quite varied life modes, from benthic (bottom-dwelling) or nektobenthic (near-bottom) to to pelagic (open ocean-dwelling, often widespread) forms, judging from the distribution patterns of their fossils.

Diversity

The highly diversified morphology of conodonts suggests that they must have once occupied most major niches in the ocean. Currently there are more than 1500 species belonging to more than 50 families reported, and there are probably many more still waiting to be discovered. With rapid radiation and diversification after their first appearance in the Late Cambrian, their diversity reached its apex in the Ordovician, with more than two-thirds (35) of the total families recorded. After the Carboniferous, conodonts experienced a gradual declining phase lasted about 100 million years until their final extinction at the end of the Triassic.

Markers for the geological time scale

Conodonts are used by geologists as an important tool for dating and correlating Palaeozoic and Triassic rocks regionally and globally because of their abundance, their highly diversified and rapidly evolved morphology and their wide distribution. They act as primary markers defining the boundaries of many systems, series and stages of the Palaeozoic and Triassic. For example, the Ordovician System can be subdivided into some 30 biozones defined by conodonts and graptolites, each of which had a time span of less than two million years. This degree of resolution is not surpassed by any other dating methods currently available.

Industrial applications

Apart for their usefulness in time resolution and biostratigraphic correlation, conodonts are also widely used in the reconstruction of the oceanography, climate, and biogeography of the Palaeozoic world, and in the practice of geological mapping and mineral explorations. For example, the colour patterns (CAI – Color Alteration Index) of conodonts are often used to determine the post-burial thermal history of sedimentary basins in the hydrocarbon exploration industry.