How To Salt a Fossil Fish

During the Pennsylvanian (323.2±0.4 – 298.9±0.15 Ma), lush tropical forests covered much of equatorial North America and Europe and were repeatedly flooded by glacial-induced sea-level fluctuations. These flooded basins were inhabited by communities of fish who used these new environments as nurseries for their young, while also exploiting nutrients from terrestrial detritus. The fish communities were diverse and mostly composed of filter-feeding acanthodians, large predatory sarcopterygians (lobe-fined fish) and a group of sharks called the xenacanths, which had bi- and tri-cuspid teeth and long eel-like bodies that helped them manoeuvre between the tree stumps of the coal swamps (Fig 1).

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Figure 1. Sketch of the xenacanth shark Orthacanthus compressus showing a tri-cuspid tooth (adapted from Ó Gogáin et al., 2016).

Two competing hypotheses exist on the physiology and habitat of these fish. The traditional hypothesis considers the fish as being only able to tolerate a small range of salinities (stenohaline) making them endemic to freshwater swamps, rivers and lakes of the Pennsylvanian. This idea is based primarily on these fish being preserved either in association with or within coal seams. The second hypothesis suggests that the fish were able to tolerate a wide range of salinities (euryhaline) and could freely migrate between freshwater and marine environments. Interpreting the different degrees of salt tolerance within these fish groups is important to understanding how early fish dispersed and colonised freshwater environments. The debate is particularly lively when discussing the xenacanths, due to the high a frequency of their fossil teeth and spines being preserved. These teeth records have allowed researchers to use numerous methods, from Sr-istopes to comparing growth bands in spines to lunar cycles, in order to distinguish the type of environment these sharks inhabited.

Pennsylvanian fish assemblages from four lithologies within the Minto Formation, New Brunswick, Canada have shed some new light on the physiology of these fish taxa. Using invertebrates and sedimentological features, the typical salinity of the lithologies could be determined and ranked in decreasing salinity as follows, lithology 1 is a coastal marine limestone containing marine brachiopods and echinoderms, lithology 2 is a brackish embayment limestone containing juvenile spirorbiform microconchids, lithology 3 and 4 are a brackish-freshwater estuarine siltstone and mudstone. When fish taxa are mapped across the lithologies (Fig 2) it becomes apparent that the majority of taxa are present in more than one salinity grade. This observation suggests that many of the taxa were euryhaline and could tolerate waters with different salinity concentrations. Orthacanthus compressus, a xenacanth shark, is found in all salinity grades indicating a euryhaline adaption within the xenacanths.

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Figure 2. The presence/absence of ten fish taxa through four lithologies from the Minto Formation.

The fish assemblages from the Minto Formation share similar taxa with other Pennsylvanian fish assemblages within the Maritimes Basin of eastern Canada. Some taxa, such as Ctenodus interruptus, Strepsodus sauroides and Archichthys portlocki are also present in assemblages in Ireland, Britain and mainland Europe. A euryhaline adaption in these fish allowed them to migrate between brackish habitats via the marine realm, similar to modern day salmon and the bullshark Carcharhinus leucas. This lifestyle explains the cosmopolitan nature of brackish to freshwater Pennsylvanian fish assemblages.

Such a euryhaline adaption may have evolved separately within each of the fish groups in response to increases of fresh meltwater during the deglaciation or in order to exploit the new habitats in freshwater to brackish inland basins. Similar adaptions are seen in the Pleistocene fish Galaxias maculatus that adapted a larger tolerance to different salinity grades during a period of deglaciation allowing them to colonize freshwater  lakes (Ruiz-Jarabo et al. 2016).

The Pennsylvanian was a period when the first complex vertebrate ecosystems were becoming established within forests linked with large water bodies. A euryhaline adaption in fossil fish allows us to understand how these fish colonised freshwater habitats leading to a better understanding of the evolution of early terrestrial ecosystems.

Specimens for this study are currently on display at the New Brunswick Museum.

By Aodhán Ó Gogáin, Ph.D. student, Trinity College Dublin. 

This research was funded by the Bob Savage Memorial Fund (University of Bristol) and from a joint grant scheme provided by the New Brunswick Museum and the New Brunswick Department of Natural Resources.

References 

Ruiz-Jarabo, I., González-Wevar, C. A., Oyarzún, R., Fuentes, J., Poulin, E., Bertrán, C. and Vargas-Chacoff, L. 2016. Isolation driven divergence in osmoregulation in Galaxias maculatus (Jenyns, 1848) (Actinopterygii: Osmeriformes). PLOS. Link to paper.
Ó GOGÁIN, A., FALCON-LANG, H. J., CARPENTER, D. K., MILLER, R. F., BENTON, M. J., PUFAHL, P. K., RUTA, M., DAVIES, T., HINDS, S. J. and STIMSON, M. R.  2016. Fish and tetrapod communities across a marine to brackish salinity gradient in the Pennsylvanian (early Moscovian) Minto Formation of New Brunswick, Canada, and their palaeoecological and palaeogeographical implications. Palaeontology, 59, 689-724.

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