Growth interruptions in silicified conifer woods from the Upper Cretaceous Two Medicine Formation, Montana, USA: implications for palaeoclimate and dinosaur palaeoecology

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Abstract

Taphonomic studies suggest that most dinosaur bone assemblages in the Upper Cretaceous (Campanian) Two Medicine Formation of Montana originated from drought-induced mass mortalities on the floodplain. This hypothesis is tested further by studying intra-annual growth patterns in silicified woods of cupressaceous/taxodiaceous conifers found in association with dinosaur bone material at a palaeolatitude of 48°N. True growth rings are completely absent, but growth interruptions characterised by concentric, variably persistent zones, 1–8 latewood cells wide are common. Interruptions are irregularly spaced so that, for example, four interruptions may occur along a radius of only 25 cells in one specimen, whilst in another specimen a radius more than 650 cells wide may completely lack interruptions. Irregular growth patterns like this occur in Upper Cretaceous woods across a broad latitudinal belt in the US Western Interior demonstrating that they resulted from regional climatic forcing rather than localised phenomena. Growth interruptions record the occurrence of short-term, aperiodic disturbances to tree growth, under an equable, megathermal climate that was otherwise favourable for continuous, year-round cambial activity. The only probable cause of growth interruption formation were drought-induced fluctuations in the water table and resultant water stress. Growth patterns suggest that sufficient water for uninhibited growth was continuously available for years at a time, but when water stress did occur, it did so repeatedly over a very few months. Comparison with present-day East Africa, where trees show very similar growth patterns, implies that the equable, but erratically humid, environment of the Late Cretaceous Western Interior was exactly the kind of setting where mass mortalities amongst megafauna might be expected. Like East Africa, this environment would have had sufficient carrying capacity to permit large herds of megafauna to develop under normal conditions, but when repeated phases of droughts struck, such populations would have been highly vulnerable.

Introduction

The Upper Cretaceous Two Medicine Formation of northwest Montana, USA, ranks amongst the world’s most productive dinosaur-bearing formations (Rogers, 1997). Dinosaur remains have been described from this fluvio–deltaic unit since the beginning of the twentieth century (Gilmore, 1914, Gilmore, 1917), and at present at least 14 genera of dinosaurs are known from the Formation (Rogers, 1990, Varricchio and Horner, 1993, Sampson, 1995). The most spectacular discoveries have been of a number of dinosaur nests which preserve both hatched and unhatched eggs containing intact embryos, and associated bones of juvenile hatchlings of the hadrosaur Maiasaura (Horner and Makela, 1979, Horner, 1982, Horner and Currie, 1994). In addition, a large number of low-diversity bonebeds dominated either by hadrosaurs, lambeosaurs or ceratopsians have been excavated (Gilmore, 1917, Rogers, 1990, Varricchio and Horner, 1993), together with one higher-diversity bone assemblage containing both iguanodontoids and theropods (Varricchio, 1995). Many of these bonebeds are parautochthonous as indicated by the degree of skeletal articulation (Varricchio and Horner, 1993).

These bone assemblages have provided tantalising insights into many aspects of dinosaur ecology ranging from their social structure (Horner and Makela, 1979, Horner, 1982) to their thermoregulation (Barrick et al., 1996). Lately, there has been considerable interest in the taphonomy of this material, and the palaeoecological implications of these data. Whilst some of the hadrosaur nesting sites may have been instantaneously buried by volcanic ash falls (Horner, 1994) or by clastic sediments from fluvio–deltaic crevasse splay events (Lorenz and Gavin, 1984), the larger majority of the bonebed material is believed to represent the parautochthonous remains of drought-induced mass mortalities on the floodplain (Rogers, 1990, Varricchio and Horner, 1993, Varricchio, 1995). The evidence for drought-induced mass death is fourfold (cf. Shipman, 1975). First, much of the bone material occurs in the fine-grained deposits of inactive river channels interpreted as waterholes (or oxbow lakes), left following cessation in fluvial discharge. Second, the associated sedimentary sequence contains calcrete palaeosols indicating a seasonally dry climate (Goudie, 1983). Third, the vertebrate assemblages contain a preponderance of sub-adult animals, the age class most vulnerable to drought-mortality according to recent studies (Conybeare and Haynes, 1984, Rogers, 1990). Fourth, most skeletal assemblages are pauci-specific or mono-specific. This is a feature common to many modern vertebrate assemblages resulting from drought-induced mortalities related to the varying susceptibility of different species to adverse conditions (Conybeare and Haynes, 1984, Rogers, 1990, Schwartz and Gillette, 1994).

Nevertheless, direct evidence for frequent and sustained droughts during the deposition of dinosaur-bearing units in the Two Medicine Formation is still lacking. The best way of analysing palaeoclimate on a sufficiently fine scale to detect drought events is by studying growth rings in fossil woods (Creber, 1977, Creber and Chaloner, 1984, Francis, 1984, Falcon-Lang, 1999). Under environmentally favourable conditions, wood is continuously deposited centripetally by the vascular cambium of trees. However, at the onset of unfavourable conditions such as low winter temperature, drought, or insect defoliation, cambial activity ceases, resulting in the formation of a growth ring in the wood marked by a concentric layer of higher density secondary xylem tissue termed latewood (Fritts, 1976). Wood therefore represents an important palaeoenvironmental archive, providing a continuous intra-annual record of what growing conditions were like in the vicinity of the tree over a period of decades to centuries (Creber, 1977, Creber and Francis, 1999). In this paper, I describe and analyse the patterns of growth seen in fragments of anatomically preserved wood from the Two Medicine Formation to analyse the Late Cretaceous climate of northwest Montana, and to further test the hypothesis that dinosaur skeletal accumulations resulted from drought-induced mass mortalities.

Section snippets

Geological setting

The Upper Cretaceous Two Medicine Formation is a ∼600-m-thick sedimentary package, exposed for approximately 220 km in highland plains to the east of Glacier National Park in northwestern Montana (Fig. 1). It represents the most proximal part of a mollasse-type, non-marine wedge deposited between the actively rising Elkhorn Mountain Range to the west and the meridionally orientated Western Interior Seaway to the east, at a palaeolatitude of 48°N (Lorenz and Gavin, 1984, Golonka et al., 1994,

Materials and methods

Standard 6×4-cm petrographic thin sections were prepared for the six best-preserved wood fragments, orientated along three planes, radial longitudinal section (RLS), tangential longitudinal section (TLS) and transverse section (TS). These specimens were assigned the accession numbers TMF1–6, and are now stored in the Palaeontology Research collection of the Department of Earth Sciences, Dalhousie University, Canada. The thin sections were described in detail with the aid of an Olympus BH-2

Analysis of wood growth patterns

True growth rings, defined here as being continuous around the tree’s circumference and having a distinctly asymmetric boundary (Fritts, 1976, Creber and Chaloner, 1984), are completely absent in the Two Medicine Formation woods. However, the woods do contain a large number and wide variety of what Schweingruber, 1992, Schweingruber, 1996 has termed ‘growth interruptions’ or ‘growth zones’, features most commonly seen in low latitude trees today. In order to understand the nature of the climate

Interpretation of growth patterns

In summary, growth features in the Two Medicine Formation are quite unlike true growth rings exhibited by most present-day temperate-zone conifers (cf. Greguss, 1972), but bear a close resemblance to what Schweingruber (1992) has described as growth interruptions in subtropical and tropical woods. In addition, these Late Cretaceous growth interruptions are not geographically localised phenomena but in fact characterise a broad latitudinal belt along the Western Interior Seaway. The

Discussion

In summary, growth patterns in Late Cretaceous woods of the Western Interior suggest the operation of megathermal climates punctuated by erratic seasonal rainfall. The only other site documented in the fossil record that contains abundant woods with very similar patterns of growth to those documented from the Two Medicine Formation, and surrounding areas, is the Upper Triassic Chinle Formation of Arizonia, USA (Ash and Creber, 1992). Growth interruptions at this site have been interpreted as

Acknowledgements

I gratefully acknowledge a Killam Post-doctoral Fellowship from Dalhousie University. I thank Gareth Dyke (University College Dublin) for collecting the fossil woods used in this study, and Peter Gasson at the Jodrell Laboratory of the Royal Botanical Gardens, Kew, for making available modern wood thin sections for study. This paper has greatly benefited from discussions with Geoff Creber (University College London) over several years, and from the reviews by Jenny Chapman (Cambridge

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