The eyes of Tullimonstrum reveal a vertebrate affinity

Abstract

*Corresponding authorswith Tullimonstrum being a notable anomaly in this respect. Tullimonstrum, a monotypic taxon known from several hundred specimens, is preserved as stains with some relief within Mazon Creek siderite nodules. Despite the uncertainty about its position in the tree of life, there is a surprisingly high level of agreement regarding the arrangement and shape of anatomical features ( Fig.1 and Table. 1). The anatomical complexity, evident cardinal axes and the bilateral symmetry demonstrates that Tullimonstrum is a bilaterian 1-3,5 but beyond this, it has defied systematic placement. Given the consensus regarding the shape and anatomical disposition of body parts this might seem perplexing but the issue is in fact quite simple: there is little agreement about its affinities because no study has identified unequivocal homologies/synapomorphies upon which to base a solid comparative anatomical interpretation. This is a classic example of how, without the criterion of topological relations between body parts as a potential falsifier of character hypotheses, testing of alternative hypotheses becomes problematic 9,10 . Different choices of extant anatomical comparator result in radically different hypotheses of homology and affinity for Tullimonstrum ( Table. 1) but evidence to test which hypothesis is correct remains elusive.Where topological data in fossils are equivocal, other homology criteria, normally subordinate to topology assume greater importance 9-11 . Here we apply the criterion of the intrinsic properties of body parts (also referred to as 'special qualities'' 10 or 'correspondence of composition' 11 ) allowing us to resolve the phylogenetic placement of Tullimonstrum.One of the defining characters of Tullimonstrum is the transverse bar. Associated with this in many specimens is a pair of dark structures which, regardless of the orientation of the fossil, occur at the distal ends of the bar (Figs 1-2; Extended Data Fig. 1). The transverse bar is relatively straight, although it bends forwards or backwards in some specimens 3 ; it is preserved in relief, suggesting a relatively recalcitrant structure, but there is no evidence that it was biomineralised 3 . Scanning electron microscopy and EDS reveal that the dark structures comprise thick, multi-layered masses of tightly-packed, micron-sized bodies composed of carbonaceous material (Fig. 2). They exhibit two distinct morphologies: The composition, anatomical localisation and fabrics indicate that the cylindrical and oblate bodies are layers of melanosomes; the range of shape and size compares closely with extant and fossilised melanosomes 6 . To further test this hypothesis we employed TOF-SIMS and principal component analyses (PCA) to compare the relative intensity distribution of the melanin-specific peaks originating from fresh, artificially matured, fossil melanin and non-melanin samples (Extended Data Fig. 3). Spectra from Tullimonstrum and pure melanin samples 12 shows a similar spectral composition (Fig. 3, Extended Data Fig.3). PCA...

Pigmented anatomy in Carboniferous cyclostomes and the evolution of the vertebrate eye

“…Pigments also occur in association with light-sensing organs in invertebrate chordates, but we can reject the hypothesis that the pigmented structures in Myxinikela and Mayomyzon are similar to the light-detecting ‘eye’ of non-vertebrate chordates because they are paired, comparatively large and comprise a mass of melanosomes of dual morphology characteristic of vertebrates [8]. Mayomyzon also preserves evidence of a lens.…”

“…This is the configuration of melanosomes in the vertebrate retinal pigmented epithelium (RPE): more cylindrically shaped melanosomes occur in the apical part, and melanosomes of oblate shape in the basal layer (see below). An identical arrangement of microbodies to those in Mayomyzon occurs in the eyes of osteichthyan fish taxa from the Mazon Creek, including Elonichthys peltigerus , Platysomus circularis and Esconicthys apopyris [8]. In other Lagerstätte, heterogeneous microbodies confined to fossil fish eyes are also interpreted as melanosomes of the RPE [12,13,21].…”

A critical appraisal of appendage disparity and homology in fishes

“…Based on a reanalysis of the character matrix fromMcCoy et al (2016) with some characters rescored,Sallan et al (2017) suggest that a non-vertebrate assignment for Tullimonstrum is more likely. Notwithstanding the uncertainty in its phylogenetic placement among stem lampreys, Tullimonstrum possesses a single long and low dorsal fin, as well as an asymmetrical oblanceolate caudal fin(Clements et al, 2016;McCoy et al, 2016).The interrelationships among lampreys, hagfishes and gnathostomes have been debated for many years, and two competing hypotheses have been proposed: (1) either lampreys and hagfishes form a clade called the cyclostomes(Duméril, 1806;Schaeffer & Thomson, 1980;Shimeld & Donoghue, 2012), (2) or hagfishes are craniates while lampreys are vertebrates, making the 'cyclos-…”

Prehistoric animals, in living color

“…Using electron microscopy and ToF-SIMS, the team identified melanosomes in the creature's eyes, arranged in layers typical of vertebrates. The team concluded that the Tully monster was a vertebrate, albeit an unusual one (13).…”