[Palaeontology, Vol. 61, Part 4, 2018, pp. 511–541] COMPUTED TOMOGRAPHY SCANNING AS A TOOL FOR LINKING THE SKELETAL AND OTOLITH-BASED FOSSIL RECORDS OF TELEOST FISHES by WERNER SCHWARZHANS1,HERMIONET.BECKETT2 ,JASOND. SCHEIN3 and MATT FRIEDMAN2,4 1Zoological Museum, Natural History Museum of Denmark, Copenhagen, Denmark; [email protected] 2Department of Earth Sciences, University of Oxford, Oxford, UK; [email protected] 3Bighorn Basin Paleontological Institute, Willow Grove, PA USA; [email protected] 4Department of Earth & Environmental Sciences, & Museum of Paleontology, University of Michigan, Ann Arbor, MI USA; [email protected] Typescript received 11 August 2017; accepted in revised form 27 November 2017 Abstract: Micro-computed tomography (lCT) scanning that do not appear in the body fossil record until the Palaeo- now represents a standard tool for non-destructive study of gene. This striking example of convergence suggests con- internal or concealed structure in fossils. Here we report on straints on otolith geometry in pelagic predators. The otoliths found in situ during routine lCT scanning of three- otoliths of Apateodus show a primitive geometry for aulopi- dimensionally preserved skulls of Palaeogene and Cretaceous forms and lack the derived features of Alepisauroidea, the fishes. Comparisons are made with isolated otolith-based lizardfish clade to which the genus is often attributed. In situ taxa to attempt correlations between the body fossil and oto- otoliths of Early Cretaceous fishes (Apsopelix and an uniden- lith fossil records. In situ otoliths previously extracted tified taxon) are not well preserved, and we are unable to mechanically from specimens of Apogon macrolepis and Den- identify clear correlations with isolated otolith morphologies. tex laekeniensis match our lCT models. In some cases, we We conclude that the preservation of otoliths suitable for find a high degree of congruence between previously inde- lCT scanning appears to be intimately connected with the pendent taxonomic placements for otolith and skeletal taphonomic history, lithological characteristics of surround- remains (Rhinocephalus, Osmeroides, Hoplopteryx). Unexpect- ing matrix, and syn- and postdepositional diagenetic effects. edly, in situ otoliths of the aulopiform Apateodus match iso- lated otoliths of Late Cretaceous age previously interpreted Key words: Teleostei, lCT scanning, otolith, Apateodus, as belonging to gempylids, a group of percomorph fishes Osmeroides, Rhinocephalus. W ITH a diversity of species challenging that of all tetra- this remarkable anatomical archive. Such intact fossils can pod radiations combined, teleost fishes are critical com- yield osteological data comparable to that available for ponent of modern vertebrate biodiversity. Extant teleosts extant species, and can be critical for illuminating patterns occupy aquatic settings ranging from ocean trenches to of character evolution (Friedman 2008) or resolving phy- alpine streams, and show a striking range of anatomical logenetic relationships (Grande 2010). Articulated speci- innovations reflecting a broad range of ecologies (Nelson mens also provide the substrate for functional and et al. 2016). Diversity in the modern fauna is comple- palaeobiological analysis, ranging from individual anatom- mented by a rich fossil record, which, for the teleost ical systems (e.g. jaws: Bellwood et al. 2015) to overall total-group, is well established since the Early Jurassic geometry of the body and fins (e.g. Friedman 2010). and extends deep into the Triassic (Friedman 2015; Despite these clear strengths, the body fossil record of tel- Tintori et al. 2015). eosts suffers from conspicuous deficiencies. The preserva- Teleosts, and indeed fishes generally, are unusual tion of articulated fishes requires particular taphonomic among vertebrates in having a fossil record characterized conditions, with horizons yielding complete skeletons by a relative abundance of articulated, effectively complete generally restricted to specific facies such as laminated skeletons. The bulk of our understanding of the relation- limestones and anoxic shales. The result is a highly ships and diversification of extinct fishes, from the semi- heterogenous, gap-filled stratigraphic distribution of nal work of Agassiz (1833–1844) onward, derives from exceptional deposits (e.g. apparent abundance during © The Palaeontological Association doi: 10.1111/pala.12349 511 512 PALAEONTOLOGY, VOLUME 61 sea-level highstands: Guinot & Cavin 2015) that, com- to marrying these two complementary teleost fossil bined with the less informative nature of isolated fish records. The most recent census suggests that fewer than bones, has led to repeated suggestions that the fish fossil 100 skeleton-based fossil species bear otoliths (Nolf record is particularly prone to bias arising from so-called 2013). In many of these taxa otoliths are mentioned Lagerst€atten effects (Patterson & Smith 1987, 1989; Patter- rather than described in detail (but see: Fedotov 1976; son 1993a, b; but see Lloyd & Friedman 2013). Schwarzhans 2014; Prikryl et al. 2017; Schwarzhans et al. Skeletal fossils are not the only palaeontological line of 2017a–e), and most of those in situ finds are from Oli- evidence bearing on past patterns of fish diversity; they gocene or younger deposits. So far few are recorded are joined by a rich record of isolated otoliths. Informally from the Eocene, and none from the Paleocene and known as ‘ear stones’, otoliths are consolidated aragonitic Late Cretaceous (Fig. 1). The Late Cretaceous–Palaeo- bodies housed within the labyrinth organ of fishes, and gene represents a crucial interval in the evolution of are involved in hearing and balance. Living actinoptery- modern teleosts, associated with the origin of many gians have three pairs of otoliths, with the largest typically extant lineages, especially within the species-rich perco- located in the sacculus. First systematically described by morph radiation (Near et al. 2013). In addition, multi- Koken (1884), otoliths are often diagnostic for species, ple skeleton-based lineages of teleost fishes became genera and higher taxa. They are abundant in the fossil extinct at or near the Cretaceous–Palaeogene boundary record where aragonite is preserved and sediments are (Friedman 2009). Otoliths are virtually unknown for unconsolidated. The otolith record is, as a consequence, these once abundant extinct groups, posing a substantial denser both stratigraphically and geographically than the problem for the interpretation of isolated otoliths from spottier archive of skeletal remains. The ubiquity of fossil the Cretaceous. otoliths led Patterson (1977, p. 580) to conclude that the Heavily compressed skeletons are the most common fossil records of teleosts and mammals were comparable articulated remains in the fish record, and it is from such in terms of quality and density: ‘whereas mammals have specimens that the vast majority of in situ otoliths have teeth, teleosts have otoliths’. been described. However, the geometry of such specimens The otolith record is not without problems. Facies biases is often a challenge for standard tomographic studies aside, otoliths are relatively uncommon in Mesozoic rocks, (although laminography might represent a productive a probable consequence of ‘calcite sea’ chemistry that alternative in the future: Sutton 2008). There are a hand- favoured the dissolution of aragonite during this interval ful of cases of otoliths reported from three-dimensionally (Nolf 1995; similar patterns apply to aragonitic fossils in preserved fish fossils (e.g. the Eocene ophidiiform other intervals characterized by comparable marine chem- Ampheristus: Stinton 1966; Schwarzhans 2007a), and istry: Palmer et al. 1988; Cherns & Wright 2000). Reports while three-dimensional preservation is often associated of Palaeozoic otoliths are sparse (Nolf 2013), reflecting little with famous Lagerst€atten (Maisey 1991; Long & Trinajstic research effort and rarity of unconsolidated sediments of 2010), horizons yielding fully inflated fish crania are dis- this age. These taphonomic and research biases are minor tributed throughout the nearly 450 million year fossil his- in comparison to the greatest challenge of isolated fossil tory of jawed vertebrates (e.g. Zhu et al. 2013; Friedman otoliths: their allocation to genera is almost exclusively & Giles 2016). Here we report on the efficacy of high- based on correlation with extant otoliths, rather than asso- resolution micro-computed tomography (lCT) scanning ciation with diagnostic fossil skeletal remains. This phenetic for isolating otoliths from three-dimensionally preserved exercise becomes more problematic for otoliths from fossil fish crania. Our survey focuses on the Cretaceous– increasingly ancient strata. The taphocoenoses of otoliths Eocene interval, and examines material from a variety of and articulated skeletons diverge as a consequence of min- host lithologies including sands, chalks and clays. eralogical differences, a restrictive range of sedimentary set- Although the quality of preservation varies considerably tings conducive to the preservation of articulated skeletons between specimens, sufficient detail is provided to allow and otoliths, and difficulties in retrieving in situ otoliths us to make comparisons with previously described oto- from articulated skeletons. Because few otoliths are known liths and to test their past taxonomic assignment using in situ from body fossils, the taxonomy of fossil otoliths