Wear, tear and systematic repair: testing models of growth dynamics in conodonts with high-resolution imaging

Abstract

Conodont elements are the earliest mineralized vertebrate dental tools and the only ones capable of extensive repair. Two models of conodont growth, as well as the presence of a larval stage, have been hypothesized. We analysed normally and pathologically developed elements to test these hypotheses and identified three ontogenetic stages characterized by different anisometric growth and morphology. The distinction of these stages is independently corroborated by differences in tissue strontium (Sr) content. The onset of the last stage is marked by the appearance of wear resulting from mechanical food digestion. At least five episodes of damage and repair could be identified in the normally developed specimen. In the pathological element, function was compromised by the development of abnormal denticles. This development can be reconstructed as addition of new growth centres out of the main growth axis during an episode of renewed growth. Our findings support the model of periodic retraction of elements and addition of new growth centres. Changes in Sr content coincident with distinct morphology and lack of wear in the early life stage indicate that conodonts might have assumed their mature feeding habit of predators or scavengers after an initial larval stage characterized by a different feeding mode.

Growth allometry and dental topography in Upper Triassic conodonts support trophic differentiation and molar-like element function

“…In M. communisti , as well as in E. rigoi , DNE values increased linearly with body size, where element length was used as a proxy for body size (Zhang et al, 2018; Ginot & Goudemand, 2019). This does not contradict the possible presence of a larval stage, during which conodonts had a different method of feeding (Shirley et al, 2018), because all specimens that are considered here were more mature.…”

“…Patterns interpreted to by produced by shearing on the surface and repaired during the animal’s growth suggest macrophagy (Purnell, 1995; Donoghue & Purnell, 1999b; Shirley et al, 2018). An active, predatory lifestyle is supported by the discovery of a conodont specimen with preserved extrinsic eye musculature (Gabbott, Aldridge & Theron, 1995), which was interpreted as indicative of conodonts having pattern vision (Purnell, 1994).…”

“…Conodont elements were retained throughout the life of an individual (Donoghue & Purnell, 1999b), recording periodic growth through apposition of crown tissue lamellae (Zhang, Aldridge & Donoghue, 1997; Dzik, 2008). Shirley et al (2018) suggested that tooth-like function in conodont elements may have only developed after a larval stage, during which they exhibited a different feeding habit, as mechanical wear of conodont elements is only present after a certain stage of ontogenetic development. After this stage, elements appear to have had prolonged intervals of dental use and short intervals of growth, during which conodonts did not feed and their elements were covered in soft tissue, depositing new layers (Bengtson, 1976; Donoghue & Purnell, 1999b; Shirley et al, 2018).…”

Evidence of parallel evolution in the dental elements ofSweetognathusconodonts

“…Based upon this fact, a 2D approach could assure positional and functional homology between all of the specimens was followed, especially as this is the first attempt to quantify shape variation in Sweetognathus conodonts. Second, the usage of histological sections identical in orientation and morphology to the cross sections used in this study, and of which are being used in finite element analysis for other conodont taxa [7,10,34], but their shape is periodically restored through a mechanism unique to conodonts [6]. Here, some degree of wear has likely changed the overall profile of the denticles (see D1 in figure 2d).…”

Ontogenetic variability in crystallography and mosaicity of conodont apatite: implications for microstructure, palaeothermometry and geochemistry

“…The ratio of nanocrystal to organic matrix varies between the three tissue types with white matter typically being the most crystal rich (or behaving as a single crystal) and containing the least amount of organic matrix. Whereas the significance of phylogeny and ontogeny in microstructure has been well documented [22,26,27], many studies of conodont crystallography combine crystallographic data from different species, different element types, different intervals of Earth history and different stages of ontogenetic development [24,[28][29][30]. The net result is aggregated data that do not display significant trends and a conclusion that conodont bioapatite crystallography is essentially identical across the clade.…”