scholarly journals Problematica old and new

2008 ◽  
Vol 363 (1496) ◽  
pp. 1503-1512 ◽  
Author(s):  
Ronald A Jenner ◽  
D.Timothy J Littlewood
Keyword(s):  
Molecular Phylogenetics ◽  
Body Plan ◽  
Phylogeny Reconstruction ◽  
Phylogenetic Placement ◽  
Genomic Markers ◽  
Body Plan Evolution ◽  
Metazoan Phylogeny

Problematica are taxa that defy robust phylogenetic placement. Traditionally the term was restricted to fossil forms, but it is clear that extant taxa may be just as difficult to place, whether using morphological or molecular (nucleotide, gene or genomic) markers for phylogeny reconstruction. We discuss the kinds and causes of Problematica within the Metazoa, as well as criteria for their recognition and possible solutions. The inclusive set of Problematica changes depending upon the nature and quality of (homologous) data available, the methods of phylogeny reconstruction and the sister taxa inferred by their placement or displacement. We address Problematica in the context of pre-cladistic phylogenetics, numerical morphological cladistics and molecular phylogenetics, and focus on general biological and methodological implications of Problematica, rather than presenting a review of individual taxa. Rather than excluding Problematica from phylogeny reconstruction, as has often been preferred, we conclude that the study of Problematica is crucial for both the resolution of metazoan phylogeny and the proper inference of body plan evolution.

The Fossil Record of the Pancrustacea

2020 ◽  
pp. 21-52 ◽  
Author(s):  
Thomas A. Hegna ◽  
Javier Luque ◽  
Joanna M. Wolfe
Keyword(s):  
Fossil Record ◽  
Hydrothermal Vents ◽  
Molecular Phylogenetics ◽  
Imaging Techniques ◽  
Extant Species ◽  
Evolutionary Studies ◽  
Molecular Phylogenetic ◽  
Terrestrial Habitats ◽  
Molecular Phylogenetic Data

Fossils are critically important for evolutionary studies as they provide the link between geological ages and the phylogeny of life. The Pancrustacea are an incredibly diverse clade, representing over 800,000 described extant species, encompassing a variety of familiar and unfamiliar forms, such as ostracods, tongue worms, crabs, lobsters, shrimps, copepods, barnacles, branchiopods, remipedes, and insects. Having colonized nearly every environment on Earth, from hydrothermal vents to terrestrial habitats, they have a diverse fossil record dating back to the Cambrian (540–485 Ma). The quality of the fossil record of each clade is variable and related to their lifestyle (e.g., free-living versus parasitic, benthic versus pelagic) and the degree of mineralization of their cuticle. We review the systematics, morphology, preservation, and paleoecology of pancrustacean fossils; each major clade is discussed in turn, and, where possible, fossil systematics are compared with more recent data from molecular phylogenetics. We show that the three epic clades of the Pancrustacea—Allotriocarida, Multicrustacea, and Oligostraca—all have Cambrian roots, but the diversification of those clades did not take place until the Middle and Late Paleozoic. We also address the potential affinities of three “problematic” clades: euthycarcinoids, thylacocephalans, and cyclids. We conclude by assessing the future of pancrustacean paleobiology, discussing new morphological imaging techniques and further integration with growing molecular phylogenetic data.

scholarly journals Naked chancelloriids from the lower Cambrian of China show evidence for sponge-type growth

2018 ◽  
Vol 285 (1881) ◽  
pp. 20180296 ◽  
Author(s):  
Pei-Yun Cong ◽  
Thomas H. P. Harvey ◽  
Mark Williams ◽  
David J. Siveter ◽  
Derek J. Siveter ◽  
...  
Keyword(s):  
Lower Cambrian ◽  
Body Cavity ◽  
Body Plan ◽  
The Body ◽  
Phylogenetic Position ◽  
Axis Formation ◽  
Large Size ◽  
Show Evidence ◽  
Body Plan Evolution ◽  
Extinct Group

Chancelloriids are an extinct group of spiny Cambrian animals of uncertain phylogenetic position. Despite their sponge-like body plan, their spines are unlike modern sponge spicules, but share several features with the sclerites of certain Cambrian bilaterians, notably halkieriids. However, a proposed homology of these ‘coelosclerites' implies complex transitions in body plan evolution. A new species of chancelloriid, Allonnia nuda , from the lower Cambrian (Stage 3) Chengjiang Lagerstätte is distinguished by its large size and sparse spination, with modified apical sclerites surrounding an opening into the body cavity. The sclerite arrangement in A. nuda and certain other chancelloriids indicates that growth involved sclerite addition in a subapical region, thus maintaining distinct zones of body sclerites and apical sclerites. This pattern is not seen in halkieriids, but occurs in some modern calcarean sponges. With scleritome assembly consistent with a sponge affinity, and in the absence of cnidarian- or bilaterian-grade features, it is possible to interpret chancelloriids as sponges with an unusually robust outer epithelium, strict developmental control of body axis formation, distinctive spicule-like structures and, by implication, minute ostia too small to be resolved in fossils. In this light, chancelloriids may contribute to the emerging picture of high disparity among early sponges.

scholarly journals The Cell's View of Animal Body-Plan Evolution

2014 ◽  
Vol 54 (4) ◽  
pp. 658-666 ◽  
Author(s):  
D. C. Lyons ◽  
M. Q. Martindale ◽  
M. Srivastava
Keyword(s):  
Body Plan ◽  
Animal Body ◽  
Body Plan Evolution

Hunting the Snark: the flawed search for mythical Jurassic angiosperms

2019 ◽  
Vol 71 (1) ◽  
pp. 22-35 ◽  
Author(s):  
Richard M Bateman
Keyword(s):  
Molecular Phylogenetics ◽  
Common Ancestor ◽  
Explanatory Power ◽  
Sister Group ◽  
Current Evidence ◽  
Phylogeny Reconstruction ◽  
Crown Group ◽  
Companion Cells ◽  
Definition Of ◽  
Cretaceous Fossils

Abstract Several recent palaeobotanical studies claim to have found and described pre-Cretaceous angiosperm macrofossils. With rare exceptions, these papers fail to define a flower, do not acknowledge that fossils require character-based rather than group-based classification, do not explicitly state which morphological features would unambiguously identify a fossil as angiospermous, ignore the modern conceptual framework of phylogeny reconstruction, and infer features in the fossils in question that are interpreted differently by (or even invisible to) other researchers. This unfortunate situation is compounded by the relevant fossils being highly disarticulated two-dimensional compression-impressions lacking anatomical preservation. Given current evidence, all supposed pre-Cretaceous angiosperms are assignable to other major clades among the gymnosperms sensu lato. By any workable morphological definition, flowers are not confined to, and therefore cannot delimit, the angiosperm clade. More precisely defined character states that are potentially diagnostic of angiosperms must by definition originate on the phylogenetic branch that immediately precedes the angiosperm crown group. Although the most reliable candidates for diagnostic characters (triploid endosperm reflecting double fertilization, closed carpel, bitegmic ovule, and phloem companion cells) are rarely preserved and/or difficult to detect unambiguously, similar characters have occasionally been preserved in high-quality permineralized non-angiosperm fossils. The angiosperm radiation documented by Early Cretaceous fossils involves only lineages closely similar to extant taxonomic families, lacks obvious morphological gaps, and (as agreed by both the fossil record and molecular phylogenies) was relatively rapid—all features that suggest a primary radiation. It is unlikely that ancestors of the crown group common ancestor would have fulfilled a character-based definition of (and thereby required expansion of the concept of) an angiosperm; they would instead form a new element of the non-angiosperm members of the ‘anthophyte’ grade, competing with Caytonia to be viewed as morphologically determined sister group for angiosperms. Conclusions drawn from molecular phylogenetics should not be allowed to routinely constrain palaeobotanical inferences; reciprocal illumination between different categories of data offers greater explanatory power than immediately resorting to Grand Syntheses. The Jurassic angiosperm—essentially a product of molecular phylogenetics—may have become the holy grail of palaeobotany but it appears equally mythical.

Postcranium of the paradigm elasmosaurid plesiosaurianLibonectes morgani(Welles, 1949)

2014 ◽  
Vol 152 (4) ◽  
pp. 694-710 ◽  
Author(s):  
SVEN SACHS ◽  
BENJAMIN P. KEAR
Keyword(s):  
Upper Cretaceous ◽  
Cervical Vertebrae ◽  
Axial Skeleton ◽  
Body Plan ◽  
Pectoral Girdle ◽  
Postcranial Skeleton ◽  
Line Drawings ◽  
Phylogenetic Placement

AbstractElasmosauridae constitutes one of the most immediately recognizable plesiosaurian radiations. Their distinctive body plan represents the popular model for Plesiosauria, and is typified by an osteological morphology especially adapted for hyper-elongation of the neck. Nevertheless, many archetypal elasmosaurids are known only from incomplete and/or inadequately documented material, a problem that has contributed to their uncertain intra-clade relationships. A prime example of this isLibonectes morganifrom the Upper Cretaceous of Texas, USA, which is frequently presented as an elasmosaurid structural proxy because of its three-dimensionally preserved holotype skull. Perplexingly though, both the taxonomic diagnosis and phylogenetic placement ofL. morganirely primarily upon the cervical vertebrae, together with the pectoral girdle and forelimb, yet most of these elements are now lost and figured only as line drawings. We therefore reviewed the remnant postcranial skeleton ofL. morganifirst-hand with the objective of clarifying its defining character states. Our observations showed that the existing diagnosis ofL. morganiis indeed inadequate. Moreover, the only identifiable autapomorphies occurred within the axial skeleton. This concurred with an examination of character scores used in published plesiosaurian phylogenies, and highlights the persistent significance of postcranial elements for discriminating elasmosaurid taxa.

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