Lasanius, an exceptionally preserved Silurian jawless fish from Scotland

Abstract

The fossil record of non-biomineralizing, softbodied taxa is our only direct evidence of the early history of vertebrates. A robust reconstruction of the affinities of these taxa is critical to unlocking vertebrate origins and understanding the evolution of skeletal tissues, but these taxa invariably have unstable and poorly supported phylogenetic positions. At the cusp between mineralized bony vertebrates and entirely soft-bodied vertebrates is the enigmatic Lasanius, a purported anaspid from the Silurian of Scotland. Interpretations of its affinity and significance are conflicted, principally because of its poorly understood anatomy due to taphonomic distortion and loss of soft-tissues. Here we use an array of modern techniques to reassess the anatomy of Lasanius via a comprehensive study of 229 complete and partial specimens. A new reconstruction clarifies the identity and position of preserved features, including paired sensory organs, a notochord, and digestive tract, supporting the vertebrate affinities of this genus. SEM-EDS trace element mapping suggests a bone-like composition of mineralized parts, but finds no evidence for mineralized dermal armour. Phylogenetic analysis recovers Lasanius as an early stem-cyclostome, and subsequent analysis supports the rejection of alternative placements (such as stemgnathostome). We highlight that while distinguishing between the early cyclostome and gnathostome condition is problematic, increasing confidence in the anatomy of key taxa, such as Lasanius, is vital for increased stability throughout the early vertebrate tree.

Multivariate mapping of ontogeny, taphonomy and phylogeny to reconstruct problematic fossil taxa

“…In both coding approaches, they are morphologically close to the decay stages of extant adult lampreys ( figure 2 d ; electronic supplementary material, figures S1, S4, S6 and S9). This closeness between fossil ‘euphaneropoids’ and lampreys, could be interpreted as affinity between the groups, providing support to several previous analyses of this group [ 45 , 66 , 77 ], although it is by no means a well-agreed-upon position [ 11 , 27 , 85 – 87 ]. As demonstrated with Palaeospondylus , model selection can have an impact on the interpretation of affinity.…”

“…The dramatic sensitivity of Palaeospondylus to different coding approaches demonstrates how comparator groups can influence the assignment of affinity of a taxon, obscuring other influences on morphological variance. Given the different implications from these two approaches to character coding, it may be more beneficial to undertake future analyses using topological-only characters, an approach encouraged for assessment of problematic taxa [ 10 , 11 , 45 ]. In any case, the approach applied here serves as a powerful visualization of these different interpretation models simultaneously in terms of coherence and influence of phylogenetic, taphonomic and ontogenetic variability.…”

“…[9,[40][41][42]), and the prevalence of problematic soft-tissue fossil taxa that have evaded classification (e.g. the 'euphaneropoids' [43][44][45] Palaeospondylus (e.g. [19,20]))…”

Colonization of the ocean floor by jawless vertebrates across three mass extinctions

“…† Gilpichthys greenei , known from numerous poorly preserved specimens from the Mazon Creek, from all of our analyses. Recent phylogenetic analyses place † G. greenei as an indeterminate jawless vertebrate [ 37 , 49 , 66 , 67 ] or as the most stemward hagfish [ 38 , 53 , 73 ]. Because † G. greenei is classically united with Myxiniformes based on the absence of features like mineralized teeth, the placement of this taxon among early vertebrates might be biased by the phenomenon of stemward slippage, whereby decomposition of the body can lead to erroneous interpretations of a fossil’s phylogenetic affinities [ 102 , 103 ].…”

“…However, the interrelationships of hagfishes and the age of living hagfish diversity remain underexplored [ 37 , 38 , 40 , 43 , 69 ]. This problem is compounded by the poor fossil record of the two living jawless vertebrate clades [ 36 – 39 , 64 – 67 , 70 – 74 ]; only a handful of fossils with well-characterized anatomy are identifiable as early total clade lampreys and hagfishes, and many fossil taxa identified as putative stem-group hagfishes and lampreys [ 75 ] may not even be jawless vertebrates [ 76 – 78 ].…”