Crypsis in the pelagic realm: evidence from exceptionally preserved fossil fish larvae from the Eocene Stolleklint Clay of Denmark

Abstract

Marine deposits of earliest Eocene age in northern Jutland, Denmark, are renowned for yielding diverse teleost assemblages that have proved central for enhancing our understanding of the early evolution of many extant actinopterygian clades. In this study, we investigate diminutive larval fish fossils from the Stolleklint Clay, Ølst Formation, that retain multiple soft‐tissue features preserved as distinct dark‐coloured stains. To examine the elemental and molecular composition of these soft parts, we employed a combination of time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), scanning electron microscopy (SEM) and energy‐dispersive x‐ray spectroscopy (EDS). Our analyses revealed that the preserved structures contain chemically identifiable eumelanin intimately associated with densely aggregated microbodies that are morphologically consistent with melanosome organelles. Thus, we conclude that the carbonaceous structures represent traces of originally melanized body parts, including the eyes and peritoneum. Comparable pigmentation patterns are seen in many extant teleost larvae that use semi‐transparency as a means of camouflage in pelagic environments, to suggest a similar visual appearance of the Stolleklint Clay fish fossils. This in turn suggests that adaptations for concealment and UV‐protection had already evolved by the beginning of the Eocene, notably during a time interval characterized by an extreme greenhouse climate, when the global fish fauna become increasingly modern in composition.

Preservation and Taphonomy of Fossil Insects from the Earliest Eocene of Denmark

“…Melanin identification was conducted by comparisons of negative ion spectra acquired specifically from dark-matter areas with reference spectra obtained from synthetic and natural variants of eumelanin. This procedure demonstrated a detailed spectral agreement both with regards to exact m/z values and the relative intensity distribution of all major peaks associated with the eumelanin molecular structure [29,34,52]. Furthermore, spectral comparisons of the fossils and eumelanin standards by PCA that included all eumelanin-related ions indicated peak intensity distributions of the beetle that are consistent with eumelanin standards (Figure S1).…”

“…In recent years, chemical evidence of eumelanin has been documented in a broad range of animal fossils from the laminated clay of the Stolleklint Clay and in calcareous concretions of the Fur Formation [5,[29][30][31]33,34]. The precise processes responsible for such a widespread presence are, however, not yet fully understood.…”

“…However, although previous work on insects from the Stolleklint Clay and Fur Formation has touched upon taphonomic and biostratinomic processes [20][21][22][23], most studies have focused on other aspects of the assemblage (e.g., [24][25][26][27][28]). Moreover, in-depth chemical and ultrastructural analyses that can provide valuable information on fossil preservation patterns have so far been conducted almost exclusively on vertebrate remains [29][30][31][32][33][34], while insects, despite their great abundance, merely have been the subject of a single investigation [5].…”

Links between Ikaite Morphology, Recrystallised Ikaite Petrography and Glendonite Pseudomorphs Determined from Polar and Deep-Sea Ikaite

“…Estimated water depth is 200-400 m (equal to 0.2 to 0.4 kilobar), based on trace fossils (ichnofacies), including bathyal zone Zoophycos in the Holmehus Clay and sublittoral Teichichnus in younger Fur Formation sediments representing a basin change presented [40,41]. Sea surface temperature (SST) obtained by Tex 86 to be 10 • C higher during the PETM onset reaching up to ∼33 • C. [42], flowed by a slow decent to post-PETM SST of 11-23 • C. Other advantages of using the Fur Formation for comparison are the excellent preservation [39,[43][44][45][46], and well-defined time frame [36,41]. Many other pseudomorph sites exist [19,20] but none with the pristine preservation of Fur formation pseudomorphs.…”