Synchrotron-aided reconstruction of the conodont feeding apparatus and implications for the mouth of the first vertebrates

Abstract

The origin of jaws remains largely an enigma that is best addressed by studying fossil and living jawless vertebrates. Conodonts were eel-shaped jawless animals, whose vertebrate affinity is still disputed. The geometrical analysis of exceptional three-dimensionally preserved clusters of oro-pharyngeal elements of the Early Triassic Novispathodus, imaged using propagation phase-contrast X-ray synchrotron microtomography, suggests the presence of a pulley-shaped lingual cartilage similar to that of extant cyclostomes within the feeding apparatus of euconodonts ("true" conodonts). This would lend strong support to their interpretation as vertebrates and demonstrates that the presence of such cartilage is a plesiomorphic condition of crown vertebrates.apical cartilage | conodont oral skeleton | early vertebrates | homology H ow the transition from "agnathans" to gnathostomes ("jawed" vertebrates) occurred is one of the most intriguing problems of evolutionary biology (1). Little is known about the endoskeleton of fossil jawless vertebrates [e.g., fossil cyclostomes (hagfishes and lampreys) and "ostracoderms"]. Although the view is still debated (2), euconodonts would have possessed the very first vertebrate mineralized skeleton in the form of their oral denticles (3, 4).The general architecture of the conodont oral skeleton is a bilaterally symmetrical array of usually 15 phosphatic elements, which generally becomes disarticulated after the decay of the supporting tissues. Hence most conodonts are known only as isolated elements. From the detailed study of hundreds of articulated "natural assemblages" and photographic simulation of their collapse, Purnell and Donoghue (5) constructed a 3D model of the Idiognathodus apparatus [presumably a template for all ozarkodinid apparatuses (6)] in which one pair of obliquely pointed M elements are located rostrally and, behind them, one unpaired S 0 (subscript number indicates distance ordering from the symmetry axis) element lying on the axis of bilateral symmetry and four pairs of elements (S 1-4 ) located on both sides of the S 0 would have grasped food and, more caudally, two pairs of pectiniform elements (P 1 , P 2 ) would have processed this food by crushing and/or slicing (5, 7, 8) ( Fig. 1 A-B) (for "standard" orientation of single elements, see Fig. S1). Purnell and Donoghue's reconstruction of a generalized resting (dead) position is very well supported and in most aspects very convincing. It is therefore adopted here as a basis upon which we build our dynamic reconstruction of the feeding apparatus at work.How could these elements actually grasp or cut prey tissues? Purnell and Donoghue's functional model (section 6 of ref. 5) was based chiefly on analogies with extant agnathans. Indeed, the "quite simple" geometry of the Idiognathodus elements does not provide much indication of what motions are possible or not [except for uncommon natural assemblages (see below)]. Thus, hypotheses were inferred from extant putative closest relatives. In our view, the more "compli...

Early Triassic conodont clusters from South China: revision of the architecture of the 15 element apparatuses of the superfamily Gondolelloidea

“…Later, the discovery of fused clusters (Ramovs 1977, 1978) and natural assemblages (Rieber 1980; Orchard and Rieber 1999) demonstrated the natural association of ‘platform’ and ramiform elements, in spite of some being regarded as of possible coprolitic origin. A more convincing natural assemblage of Neogondolella also collected by Rieber is illustrated by Goudemand et al. (2011).…”

New insights into the diversity dynamics of Triassic conodonts

“…During the early Paleozoic, conodonts showed a large variety of apparatus styles, but most of them were poorly known. In contrast, the apparatus composition of the late Paleozoic and Mesozoic conodonts is better understood, due to the relatively abundant record of natural assemblages and fused clusters of different species (Rhodes 1953;Krivic and Stojanovic 1978;Ramovs 1978;Mietto 1982;Nicoll 1983Nicoll , 1985Mastandrea et al 1999;Goudemand et al 2011Goudemand et al , 2012. These late Paleozoic and Mesozoic apparatus show a more stable architecture, with conservative ramiform elements (even at the family level) and distinctive pectiniform elements that evolved rapidly, becoming the base for the systematics of the group (Aldridge 1988).…”

Constraining the Deep Origin of Parasitic Flatworms and Host-Interactions with Fossil Evidence

“…Most authors agree that the fossil circlets of hooks probably belong to platyzoan helminths (Upeniece, 2001(Upeniece, , 2011Poinar, 2003), although we cannot entirely exclude the possibility that they belong to a now-extinct lineage of parasites. A reinvestigation of these attachment structures with particular focus on taphonomy, their composition (element analysis) and the three-dimensional structure using computer tomography can be particularly useful for disentangling phylogenetic affinity as it did for the elements of the now-extinct conodonts (Purnell and Donoghue, 1997;Goudemand et al, 2011;Murdock et al, 2013). Whatever the exact taxonomic affinity of the parasite hook circlets, they remain the oldest direct evidence for the presence of helminthegnathostome and helminthevertebrate association in the fossil record (Boucot and Poinar, 2010).…”

“…Host phylogeny was modified from Goudemand et al (2011). Note that the host gap would be considerably greater if the position of conodonts would be more basal as postulated by some authors Turner et al, 2010).…”

“…The range of the host gap between Cambrian and extant pentastomids ( Figure 5) might therefore depend on the systematic position of conodonts, which is still debated. Most authors agree that conodonts are chordates, probably either stem-or crown-vertebrates Turner et al, 2010;Goudemand et al, 2011;Murdock et al, 2013;Donoghue and Keating, 2014). Interestingly, putative pentastomid remains were recently also reported from ostracods within the Silurian Herefordshire Lagerst€ atte (Siveter et al, 2015), which further increases the host gap between extant and fossil pentastomids.…”