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

Abstract

Exceptionally preserved fossils of soft-bodied organisms provide unique evidence of evolutionary history, but they are often contentious; different approaches frequently produce radically different reconstructions of taxa and their affinities. Conflict arises due to difficulties in disentangling the three non-independent factors that underlie all morphological variation within and between fossils: ontogeny, taphonomy and phylogeny. Comparative data from extant organisms can be extremely powerful in this context, but is often difficult to apply given the multi-dimensionality of anatomical variation. Here, we present a multivariate ordination method using discrete morphological character data from modern taxa at different ontogenetic and taphonomic stages (semaphoront and ‘semataphonts’). Analysing multiple axes of morphological variation simultaneously allows us to visualize character combinations that are likely to exist in fossil specimens at intersecting stages of growth and decay, and thus constrain interpretation of fossils. Application to early vertebrates finds variation in fossil specimens to be accounted for by all three axes: primarily decay in Mayomyzon , ontogeny in Priscomyzon and phylogeny in ‘euphaneropoids’ and Palaeospondylus . Our demonstration of empirical multi-factorial variation underscores the power of multivariate approaches to fossil interpretation, especially non-biomineralized taxa. As such, this conceptual approach provides a new method for resolving enigmatic taxa throughout the fossil record.

The Avian Acetabulum: Small Structure, but Rich with Illumination and Questions

“…Partial medial occlusion of the acetabulum continues to be observed in taxa close to Neornithes (e.g., Iaceornis, the pelvis of which was assigned to Apatornis by Marsh [184]) [83] (pp. [46][47][48][49][50][51][52], though not in Ichthyornis [185]. In both enantiornithines and ornithurines, the maintenance of balance in obligately bipedal locomotion is linked to the presence of an antitrochanter [104].…”

“…Fossils present even greater challenges to faithful preservation: the quality of the preserving sediment, microbial or micro-faunal destruction of soft tissue and thin bone, compaction and water-rock interaction, differential destruction from preservation diagenesis, and weathering of the surviving fossilized material are all factors, too easily forgotten. Taphonomic factors can obscure not only anatomical interpretation, as is clear from disputes on the composition and homology of fibrous or filament-like structures in various theropod dinosaurs [25][26][27][28][29][30][31][32][33][34][35][36][37] but can also obscure phylogenetic signals in analyses that are wholly reliant on morphological (particularly paleontological) data or which attempt to integrate data types, as illustrated by the problem of "stemward slippage" in attempts to understand the origin of major phyla [38][39][40][41][42][43][44][45][46][47][48]. Given the evolutionary tendency of birds toward reduction and thinning of bone, the avian skeleton (and especially the acetabulum) is particularly susceptible to taphonomic alteration or distortion during preparation for collections.…”

The Life and Death of Jamoytius kerwoodi White; A Silurian Jawless Nektonic Herbivore?

“…It was once considered the most primitive known vertebrate [1], but with additional studies, its affinities are now debatable [3][4][5][6][7][8]. Because the interpretations of such exceptionally preserved soft-bodied fossils is difficult, observed features can be interpreted in different ways [9][10][11] (Figure 2). Various cladistic analyses of Jamoytius with other jawless vertebrates, using different character codings, give divergent results [7,[12][13][14][15][16][17][18][19][20].…”

“…Based on both morphological and molecular evidence, Brownstein and Near [32] estimated that 90% of living lamprey clades originated only since the late Cretaceous. As reconstructed by Reeves and Sansom [11], carnivorous lampreys evolved from non-carnivorous early Paleozoic forms The fossil evidence for early evolution of lampreys is limited. Based on both morphological and molecular evidence, Brownstein and Near [32] estimated that 90% of living lamprey clades originated only since the late Cretaceous.…”

“…Based on both morphological and molecular evidence, Brownstein and Near [32] estimated that 90% of living lamprey clades originated only since the late Cretaceous. As reconstructed by Reeves and Sansom [11], carnivorous lampreys evolved from non-carnivorous early Paleozoic forms in the Jurassic, when innovations of their feeding apparatus may underlie their evolutionary increase of the body size and the 'modernization' of their life-history during the Jurassic period [33], and then radiated from the late Cretaceous times (~100 Ma) and especially from Miocene times (~25 Ma) onwards into many both carnivorous and non-carnivorous forms.…”