Devonian Period

Abstract

This page describes the Devonian Period, including stratigraphy, paleogeography, and the fossil record. A famous lagerstätte – the Rhynie Chert – is briefly discussed, followed by a sketched outline of some of the major fossil groups.

Keywords: Devonian, Devonian biota, fossil record, evolution

Introduction

Fishes are the dominent animals; scale tree forests appear on land, inhabited by the first wingless insects; blastoids are at their peak; the first ammonites and amphibians evolve.

Stratigraphy

Type Section/Sections

The Devonian was proposed by Roderick I. Murchison and Adam Sedgewick in 1840. The type section is in Devonshire; its boundaries are based mainly on fossils.

Lower (Silurian-Devonian) Boundary

The base of the Devonian is defined immediately at the first appearance of the graptolite species Monograptus uniformis in the rhythmically alterating limestones and calcareous shales of ‘Bed 20’ in the Klonk Section, which is located about 35km southwest of Prague, near the village of Suchomasty, in the Czech Republic. The age is established at 410 Ma.

Upper (Devonian-Carboniferous) Boundary

GSSP for the Devonian-Carboniferous Boundary Definition: The base of the Carboniferous System is defined at the first appearance of the conodont Siphonodella sulcata within the evolutionary lineage from Siphonodella praesulcata to Siphonodella sulcata. The Devonian-Carboniferous Boundary is at the base of Bed 89 in Trench E' at La Serre, France

Location: The La Serre section is located in the southeastern Montagne Noire, Departement Hérault, District of Cabrières, southern France. Artificial trench E', which averages 80cm in depth on the southern slope of La Serre Hill, is about 125m south of the hilltop (252m), about 525m east of La Roquette farmhouse, 2.5km northeast of the village of Fontès. (sheet 1:25,000 Pézenas XXVI/44, 1-2, x = 682.55, y = 140.12).

Sedimentology: The rocks in trench E' are part of a geologic unit named "Klippen of Cabrières". The youngest Devonian and earliest Carboniferous beds are characterized by a sequence of predominantly biodetrital oolitic limestone within a pelagic matrix of shale and cephalopod bearing calcilutites.

Primary Markers: First appearance of the conodont Siphonodella sulcata within the evolutionary lineage from Siphonodella praesulcata to Siphonodella sulcata at the base of Bed 89 in trench E'.

Secondary Markers: Trilobites: Belgibole abruptirhachis, Archegonus (Phillibole) and Carbonocoryphe occur in Bed 89.

Other Locations around the World: Trilobite Belgibole abruptirhachis occurs immediately above the Hangenberg Schiefer interval in various cephalopod-bearing sections from the Renish Slate Mountains (Germany), the Holy Cross Mountains (Poland) and the Carnic Alps (Austria).

References: Paproth, E., Feist, R., and Flaijs, G., 1991. Decision on the Devonian-Carboniferous boundary stratotype. Episodes 14/4, p. 331 - 336.

Chronology

Paleogeography

Major Tectonic Events

Land and Sea

During the Devonian, there were three major continental masses: North America and Europe sat together near the equator, much of their current land underneath seas. To the north lay a portion of modern Siberia. A composite continent of South America, Africa, Antarctica, India, and Australia dominated the southern hemisphere.

(RCB paleogeographic reconstruction)

Climate

Paleontology

General Characteristics

The first terrestrial arthropods, including wingless insects and the earliest arachnids. In the oceans, brachiopods flourished; crinoids and other echinoderms, tabulate and rugose corals, and ammonites were also common.

However, the Devonian is probably most noted for the diversification of rise to prominence of various groups of fishes; indeed this period is occasionally referred to as the "age of fishes."

Tetrapods appear.

The vegetation of the early Devonian consisted primarily of small plants, the tallest being only a meter tall. By the end of the Devonian, ferns, horsetails and seed plants had also appeared, producing the first trees and the first forests. Archaeopteris is one of these first trees.

Lagerstätten

Rhynie Chert: Lr Dev: The Rhynie Chert in Scotland is a Devonian age deposit containing fossils of both Zosterophyllophytes and Trimerophytes, the two major lines of vascular plants. This indicates that prior to the start of the Devonian, the first major radiations of the plants had already happened. The oldest known vascular plants in the Northern Hemisphere are Devonian.

Hunsrück Schiefer: Lr Dev

Gilboa: M. Dev; New York State; spiders and pseudoscorpions; Shear et al. 1984, Selden et al. 1991

Major Evolutionary Events

Fishes are the dominent animals; scale tree forests appear on land, inhabited by the first wingless insects; blastoids are at their peak; the first ammonites and amphibians evolve.

Major Taxa

Annelida

Arthropoda

terrestrial arthropoda

Brachiopoda

Echinodermata

graptolites

fish

Bryophyta

higher plants

Extinctions

"The Frasnian-Famennian (F-F) faunal crisis represents one of the ‘Big Five Mass Extinctions’ in Earth History. Studies have been published that favor either a collision with an extraterrestrial bolide [e.g. 1] or changes in the oceanographic and climatic systems [e.g. 2]. However, no unequivocal evidence on the origin of the Late Devonian faunal breakdown was reported. E.g. iridium anomalies reported from Australia [3] and Southern China [4, 5] post-date the F-F boundary by 1.5 to 2 Ma. Microtekites from Belgium boundary sections [6, 7] were seen in context with the 368 ±1 Ma old Siljan Ring impact structure. However, the cosmic origin of these spherules has been questioned [8] and the microtektites were found above the F-F boundary and clearly post-date the extinction event. Negative carbon isotope excursions were interpreted as evidence for a sudden biomass crash hat may have been triggered by a bolide impact [9, 10]. In contrast, two positive d13C excursions measured in the late Frasnian and at the F-F boundary [2, 11] seem not to support the idea of a dramatic decline in primary productivity.

"The geochemical data base for the Late Devonian extinction event shows no evidence for a bolide impact as potential cause of the mass extinction. Instead, repeated changes in the carbon cycle of the ocean-atmosphere system are indicated by positive carbon excursions. The enhanced organic carbon burial is indicated by the higher d13C values, deposition of the Kellwasser horizon and a positive excursion in d34S of sulfides and organically-bound sulfur A decrease in atmospheric and oceanic dissolved CO2 contents is expected and may have culminated in global climatic cooling. A decrease in tropical seasurface temperature of 7°C is indicated by preliminary conodont and fish apatite d18O data.

"The paleontological data base seems to support the conclusion that climatic cooling may have represented a potential mechanism for the Late Devonian mass extinction [e.g.16]. Organisms living in the tropical to subtropical pelagic and shallow-water ecosystems were heavily decimated. Organisms thriving in higher latitudes or in deeper waters were only slightly affected. Further, late Frasnian faunal groups that were adapted to cooler temperatures migrated into tropical latitudes during the early Famennian. This pattern suggests that climatic cooling in conjunction with significant oceanographic changes may represent a powerful scenario to account for the Late Devonian mass extinction" (Joachimski & Buggisch: full article).

References

Davidson, R.G.; Trewin, N.H. 1999: Unusual Soft Tissue Preservation in Middle Devonian Fish-Bearing Nodule Beds. Palaeontological Association 43rd Annual Meeting, University of Manchester, 19-22 December 1999. Department of Geology & Petroleum Geology, Meston Building, University of Aberdeen, Aberdeen, Scotland AB24 3UE (bob_davidson@prosar.freeserve.co.uk).

Long, John A. 1995: The Rise of Fishes. Johns Hopkins.