Cainozoic Research, 16(1), pp. 3-12, June 2016 3 Evidence and implications of marine invertebrate settlement on Eocene otoliths from the Moodys Branch Formation of Montgomery Landing (Louisiana, U.S.A.) Gary L. Stringer Museum of Natural History, University of Louisiana at Monroe, Monroe, Louisiana 71209, USA; [email protected] Received 2 November 2015, revised manuscript accepted 24 February 2016 Microscopic analysis of 3,256 fish otoliths from the Eocene Moodys Branch Formation at Montgomery Landing, Louisiana, U.S.A., revealed 93 specimens with evidence of marine invertebrate settlements, primarily encrustings and boreholes. Although size, abun­ dance, shape (stability), durability, and surface residence­time influenced the use of otoliths, key factors were size, abundance, and surface time. Invertebrates affecting otoliths were cnidarians (scleractinian solitary corals), bryozoans (cheilostome species), molluscs (mainly gastrochaenid bivalves), and annelids (serpulids), noted by larval form settlement, encrustation, and drilling. The size of the scleractinian corals, the time duration of the serpulids, encrustation by cnidarians and serpulids and paucity of other epifauna such as bryozoans, and the lengths of Gastrochaena borings appear to indicate that the otoliths did not remain exposed on the sea floor for an extended period. Surface residence­time may also explain why the abundant, diverse invertebrates present affected only about 3% of the otoliths. KEY WORDS: fish otoliths, Eocene, invertebrates, surface residence­time. Introduction Background and previous studies The vast majority of literature on fossil otoliths has con- Otoliths are unique because they are specialised and in- centrated on taxonomy and paleoecology (Nolf, 2013). tegral components of the fish’s acoustico­lateralis system There is a substantial and notable paucity of research (Norman, 1931; Lowenstein, 1957; Lagler et al., 1962). on otoliths after their release from the fish’s neocra- Schäfer (1972) noted that otoliths are the last parts of a nium until they become fossilised. Courville & Collin dead fish to be freed because of the location of the laby- (2002) in their discussion of taphonomic sequences re- rinths in the bony neocranium, which is quite strong and ferred to the events recorded on the fossils between the durable. Fitch & Brownell (1968), Gaemers (1977), Gae- time the dead organisms settled on the seafloor and the mers & Langeveld (2015) and Nolf (1985; 2013) noted moment they become permanently incorporated into that fish otoliths can be enriched in marine bottom sedi- the sediment. ments through the excretion of piscivorous organisms such The objective of the present research was to analyse fos- as sharks, fish, and mammals. Only one study (Gaemers & sil fish otoliths to determine what evidence, if any, they Langeveld, 2015) is known that addresses the activity of in- provided of the taphonomic process from the death of the vertebrates on otoliths during taphonomy. Otoliths (n = 32) fish to fossilisation in the marine environment. Samples with drill holes, representing four species of gadid fishes and surface collections were obtained from the Eocene from a Miocene-Pliocene site in southeast Netherlands, Moodys Branch Formation at the Montgomery Landing were reported by these authors. Over 81% of the drill locality in Louisiana (U.S.A.) since abundant otoliths and holes, attributed to naticid gastropods, were found in oto- invertebrates were known from the site. In addition to liths of one gadid species. taxonomic identification, the otoliths were microscopi- cally scrutinised on the inner and outer faces as well as the margins. This analysis indicated that some of the oto- Location of collecting site liths provided clear evidence of processes occurring af- ter liberation from the skull but before fossilisation. The Otolith specimens for this study were collected in Grant indications of invertebrate settlement on the otoliths are Parish, Louisiana, U.S.A. (31.7°N, 92.9°W) at a site known described and discussed. as Montgomery Landing (Schiebout, 1986). The site is located approximately 100 m west of Creola Cemetery on 4 Stringer. Marine invertebrate settlement on Eocene otoliths from the Moodys Branch Formation Figure 1. Location of the Montgomery Landing site on the Red River near Montgomery, Grant Parish, Louisi- ana, U.S.A. the east side of the Red River (Fig. 1). Prior to channelisa- the formation is the highly abundant and diverse shelf as- tion activities on the river by the U.S. Army Corps of En- semblage, which includes micro- and macroscopic inver- gineers, the location was characterised by a cut-bank that tebrates and vertebrates (Schiebout, 1986; John, 2010). formed a cliff known as Creola Bluff. Creola Bluff was about 500 m in length and reached a maximum height of 14 m. Unfortunately, the chanlisation essentially de- Methodology stroyed the site, but bulk and surface samples previously collected were utilised for this study. Several unwashed sediment samples (2 kg and 4 kg) stored at the University of Louisiana at Monroe Museum of Natural History and a surface collection by S. Breard Stratigraphy of site were used in this study. Otolith studies typically employ much larger samples to increase the probability of recov- Three Eocene formations were exposed at the Mont- ering an as complete as possible assemblage, with sam- gomery Landing locality (Fig. 2), but only otoliths from ples exceeding 100 kg and even 1000 kg (Nolf & Dock- the middle Eocene Moodys Branch Formation were em- ery, 1990; Nolf & Stringer, 2003). The samples were wet ployed in this study, but it cannot be excluded that some screened using plain water with no additives because of were eroded from underlying rocks. The Moodys Branch their potential adverse effect on the aragonitic otoliths. Formation, which is in the Jackson Group, represents a During the wet screening, all residue that was retained marine transgression facies (Schiebout, 1986; Breard, on a U.S. Standard Sieve #30 (mesh size .595 mm) was 1986). The formation is a massive, greenish-gray marly kept for study. An Olympus binocular microscope (6.7x sand with abundant glauconite, angular quartz, and fecal to 40x) was employed to extract the otoliths from the pellets. One of the most distinguishing characteristics of residue. After identification, the otoliths were carefully Cainozoic Research, 16(1), pp. 3-12, June 2016 5 Figure 2. Stratigraphy of the Montgomery Landing locality on the Red River near Montgomery, Grant Parish, Louisiana, U.S.A. examined on the inner and outer faces as well as along of the specimens that indicated invertebrate settlement the margins for evidence related to invertebrate settle- were in two size categories (2­3.99 mm and 4­5.99 mm). ment. An extensive comparative invertebrate collection This indicated that at least some of the otoliths experi- from the site was utilised in identifying the remains on enced delayed burial during the taphonomic process and the otoliths. Otolith identification was aided by a com- were subjected to biological actions on the substrate. In- parative collection from the site developed by the late J. vertebrate activities recognised on the otoliths included Fitch (Natural History Museum of Los Angeles County) settlement by larval forms, encrustation, and drilling. as well as a comparative collection from the overlying Otoliths from ten fish taxa retained evidence of inver- Yazoo Clay supplied by D. Nolf (Royal Belgian Institute tebrate settlement, and the greatest evidence of inverte- of Natural Science, Brussels, Belgium). brates was found on the outer face of the otoliths (n = 54) followed by the inner face (n = 42). Only two remains of invertebrates were found uniquely on the margins. The Results total number exceeds 93 because some otoliths had inver- tebrate traces on both inner and outer faces. The 4 kg sample produced 2,133 otoliths, the 2 kg sam- Analysis revealed that representatives of at least four ple yielded 762 otoliths, and surface collected specimens phyla of invertebrates affected the otoliths. Invertebrate added 361 for a total of 3,256 otoliths, which is a sig- phyla included the Cnidaria, Bryozoa, Mollusca, and An- nificant number for analysis. Microscopic examination of nelida. The most commonly occurring traces on the oto- the otoliths resulted in the identification of 93 specimens liths was by cnidarians, which were recognised by sali- (2.86% of the total otoliths) that exhibited evidence of in- ent characteristics such as septa and radial symmetry. A vertebrate settlement. Otoliths from the site ranged from total of 36 remains of scleractinian corals were found on approximately 1.5 to 26.5 mm in length. However, 67% the otoliths, and all of these remains appear to represent 6 Stringer. Marine invertebrate settlement on Eocene otoliths from the Moodys Branch Formation solitary corals. Although several complete corals were to be gray to grayish-white in color and translucent, and located on the otoliths, most of the scleractinian remains might be a cement of Rotularia leptostoma (Pl. 1, fig. N). were partial, and some were just faint outlines. Thirty- four of these remains are believed to represent Discotro- chus orbignianus Milne Edwards & Haime, 1848 (Pl. 1, Discussion and implications figs ­A C), which was extremely abundant at Montgomery Landing (Breard, 1986). Two other cnidarians do not ap- The number of otoliths with recognisable traces of in- pear to be Discotrochus orbignianus and could not be vertebrate