Survival of the smallest?

Survival of the smallest?  – Trends in brachiopod size across the Late Triassic mass extinction

Overview

wp_20161213_10_05_43_proOver the summer of 2016, I co-supervised a Palaeontological Association-funded summer intern, Fiona Pye, along with Chris Hughes, Zoe Hughes, and Prof. Richard Twitchett (all of the Natural History Museum (NHM), London). Fiona was tasked with attempting to detect any evidence of a ‘Lilliput Effect’ in brachiopods following the Late Triassic mass extinction. The project took advantage of the recent curatorial improvements and digitization of the Mesozoic brachiopod collections at the NHM.

More information about PalAss funding for undergraduate interns can be found on the Palaeontological Association website.

Introduction

Body size is a key indicator of an animal’s ability to survive and reproduce. In the immediate aftermath of mass extinction events, surviving animals tend to become temporarily a lot smaller, a term referred to as the ‘Lilliput Effect’ (Twitchett 2007). Greenhouse conditions are ubiquitous with a number of Phanerozoic extinction events where marine survivors find themselves living in waters that are warmer and deoxygenated. This reduction in oxygen, coupled with the strains of higher temperatures on metabolic rates, causes species to mature faster and to smaller sizes whilst placing constraints on resources allocated to essential processes such as maintaining a hard shell (Garilli et al. 2015). The Lilliput Effect is well recognised in the aftermath of the Late Permian mass extinction (c. 252 Ma) (Twitchett 2007), but less so in the Late Triassic mass extinction (c. 201 Ma) that occurred some 50 million years later. However, given the similarities between the causal mechanisms of the two events, i.e. volcanic-induced extreme global warming, it is hypothesised that marine shelly macrofaunas will display a similar pattern of size reduction in the immediate aftermath of the Late Triassic mass extinction event. This study uses brachiopods from the Natural History Museum, London collection to investigate body volume changes, and thus the presence of a Lilliput Effect, in the order Rhynchonellida across the Triassic/Jurassic boundary.

Methods

Late Triassic-Early Jurassic Rhynchonellid brachiopod genera were sampled from the Natural History Museum collections. Specimens to be studied were limited to those within the order Rhynchonellida as this group has better studied taxonomy than other Mesozoic brachiopods (Ager 1990). Specimens were measured if they possessed undamaged principal axes and if they fulfilled criteria for appropriate chrono- and/or biostratigraphic resolution. Late Triassic specimens were recorded as either Keuperian (Carnian-Norian) or Rhaetian, and Early Jurassic specimens were recorded to ammonite zones. Specimens were measured with digital callipers along the anteposterior (A), transverse (T), and dorsoventral (D) axes (Figure 1).

figure-1

Figure 1. NHM 32415 b with principal axes indicated by white lines. A = anteroposterior; T = transverse; D = dorsoventral.

Taxonomic revisions were undertaken using The Paleobiology Database and The Treaties on Invertebrate Palaeontology, Volume H. After taxonomic updates,  specimens were left in the order Rhynchonellida. Specimen volume was then calculated using the equation from Novack-Gottshall (2008).

equation-for-website

where A = anteroposterior, T = transverse, D = dorsoventral (see Figure 1).

Mean and standard deviations of specimen volume were calculated for each genera per time bin and significance of changes in mean body size through time were calculated using two-tailed Wilcoxon tests.

Results

Mean body size across the entire order of Rhychonellida drops significantly from the Rhaetian to the Hettangian (W = 1613, p < 0.001) (Figure 1A). This trend is repeated in two genera that cross the T/J boundary: Rhynchonella (W = 92, p < 0.001) and Calcirhynchia (W = 128, p < 0.001) (Figure 2BC). In general, genera that are only present in the Triassic (e.g. Fissirhynchia) are larger than those genera that are exclusively Jurassic (e.g. Cuneirhynchia) (Figure 2D). There are no brachiopod specimens from the Planorbis or Liasicus ammonite zones, i.e. the first two zones of the Hettangian which represent the post-extinction communities, meaning the first Jurassic brachiopods post-date the extinction by roughly 1 million years. No specimens were found for the Turneri zone in the early Sinemurian.figure-2Figure 2. Mean body size (volume) of brachiopods through the Late Triassic-Early Jurassic (A) order Rhynchonellida; (B) Rhynchonella; (C) Calcirhynchia; (D) Cuneirhynchia.

Discussion

The results show that mean Early Jurassic body size was smaller than mean Late Triassic body size in Rhynchonellid brachiopods. This suggests that there may have been a Lilliput Effect associated with the Late Triassic mass extinction event as seen in the aftermath of the Late Permian event some 50 million years earlier. However, there are a number of limitations that need to be taken into account: (i) Some genera had too low a sample size to calculate statistically meaningful results, an issue largely caused by taxonomic reclassification of groups not reflected in the collection; (ii) The genus Rhynchonella is widely recognised as a wastebasket taxon and may actually represent order level trends; (iii) The absence of specimens in the collections from the two lowest Jurassic ammonite zones (Planorbis and Liasicus) makes it difficult to link the discrepancy in body size recorded between Late Triassic and Early Jurassic brachiopods to the Late Triassic extinction event; (iv) No biozones are recorded for Late Triassic specimens, so sampling is only at Stage resolution, which might mask more subtle temporal trends; (v) Disparity in sites of collection between the Late Triassic (Germany, Austria, Czech Rep.) and the Early Jurassic (mostly U.K.) specimens means that facies differences cannot be ruled out as drivers of the body size trends. It may, therefore, be that any observed Lilliput Effect in the Lower Jurassic sediments of the UK is more closely related to the frequent anoxic/dysoxic pulses that occur throughout the Early Jurassic of Western Tethys (Allison et al. 2008).

Conclusions

Rhynchonellid brachiopods display a statistically significant drop in body volume across the Triassic/Jurassic boundary. The same trend is repeated at the generic level by Rhynchonella and Calcirhynchia. In general, Early Jurassic specimens display lower mean volume than Late Triassic specimens, suggesting some evidence of a Lilliput Effect associated with the Late Triassic mass extinction but, further investigation is required in order to rule out confounding influences associated with variable biogeographic and facies effects.

Acknowledgements. Thanks to the Palaeontological Association, who funded this research through an undergraduate research bursary (PA-UB201605). Thanks to Jed Atkinson (Leeds) for methodological advice and Autumn Pugh (Leeds) for assistance with R.

REFERENCES

AGER, D. V. 1990. Monograph of the Palaeontographical Society. British Liassic Terebratulida (Brachiopoda): Part 1. Monograph of the Palaeontographical Society, Palaeontographical Society, London.

ALLISON, P. A., HESSELBO, S. P. and BRETT, C. E. 2008. Methane seeps on an Early Jurassic dysoxic seafloor. Palaeogeography, Palaeoclimatology, Palaeoecology, 270, 230-238.

GARILLI, V., RODOLFO-METALPA, R., SCUDERI, D., BRUSCA, L., PARRINELLO, D., RASTRICK, S. P. S., FOGGO, A., TWITCHETT, R. J., HALL-SPENCER, J. M. and MILAZZO, M. 2015. Physiological advantages of dwarfing in surviving extinctions in high-CO2 oceans. Nature Clim. Change, 5, 678-682.

NOVACK-GOTTSHALL, P. M. 2008. Using Simple Body-Size Metrics to Estimate Fossil Body Volume: Empirical Validation Using Diverse Paleozoic Invertebrates. PALAIOS, 23, 163-173.

TWITCHETT, R. J. 2007. The Lilliput effect in the aftermath of the end-Permian extinction event. Palaeogeography, Palaeoclimatology, Palaeoecology, 252, 132-144.

Palass

Fiona presented this work at the Palaeontological Association Annual Meeting in Lyon in December 2016. if you have any questions about this work, email Fiona (ee14f3p@leeds.ac.uk) or myself (a.dunhill@leeds.ac.uk).

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