Palaeozoic ‘conodont pearls’ and other phosphatic micro-spherules

Abstract

Sub‐millimetre‐sized phosphatic spherules are often found in acetic acid‐insoluble residues produced for microfossil extraction. As they are typically associated with conodont elements and have a similar chemical composition, they are informally known as ‘conodont pearls’. Still, the origin of these micro‐spherules has been controversial, and authors have disagreed regarding their mode of formation, or if they are biogenic or not. In this study, an assortment of micro‐spherules from several localities and stratigraphical levels were analysed using multiple methods, in an effort to shed light on the origin of these enigmatic objects: ocular investigation with a stereomicroscope, chemical analyses employing energy‐dispersive mass spectrometry, imaging through scanning electron microscope, and synchrotron radiation X‐ray tomographic microscopy. Collectively, the techniques employed allow for near‐complete characterization and description of the study specimens. At least five different groups, or morphotypes, of spherules can be discerned, which differ both in morphological and chemical details. Most specimens are notably spherical and display concentric layers of growth. Several of the specimens have a central nucleus, sometimes with one or more objects located closely together. Internal details and chemistry suggest that the phosphatic spherules probably are of different origin. Thus, the term ‘conodont pearl’ encompasses confusingly similar objects deriving from different organisms and/or processes, and only careful analysis can reveal their individual origin. The only organisms unequivocally associated with (in situ) phosphatic micro‐spherules are ceramoporid bryozoans, and the conodonts Cordylodus and Westergaardodina, but the possible function and significance of these objects remain enigmatic.

Late Devonian dimpled phosphatic microspherules associated with conodonts are possible otoliths of short-lived and opportunistic organisms

“…Recently, Lindskog et al . () outlined five types of phosphatic microspherules of possibly different origins, in which only type 1 of microspherules closely resemble ‘conodonts pearls’ as originally described by Glenister et al . (), characterized by concentric internal annuli, high degree of sphericity, a smooth exterior and a small dimple on the surface.…”

“…During the digestion of limestone samples in acetic acid, hundreds of typical type 1 microspherules sensu Lindskog et al . () or original ‘conodont pearls’ sensu Glenister et al . () were found in the residues.…”

“…If the ‘conodont pearls’ were related to conodonts or were ‘conodont otoliths’, they should be found in more or less all sample residues yielding many conodont elements (Lindskog et al . ). The interrelated occurrences between conodonts, fish teeth and phosphatic microspherules may just indicate that the animal that secreted phosphatic microspherules do not leave much to make fossils just like the conodont animals.…”

“…Lindskog et al . () redefined the ‘conodont pearls’ on the basis of morphology, internal structure and chemical analysis and stated that only type 1 microspherules meet the original definition proposed by Glenister et al . () and those ‘conodont pearls’ are characterized by concentric internal annuli, high degree of sphericity, smooth exterior and a small dimple on the surface (Stauffer ; Leuteritz et al .…”

“…; Huang & Gong ; Lindskog et al . ). Despite the fact that some researchers noticed internal growth annuli within phosphatic microspherules (Leuteritz et al .…”