scholarly journals A primitive starfish ancestor from the Early Ordovician of Morocco reveals the origin of crown group Echinodermata

2017 ◽  
Author(s):  
Aaron W. Hunter ◽  
Javier Ortega-Hernández
Keyword(s):  
Phylogenetic Analyses ◽  
Radial Symmetry ◽  
Early Evolution ◽  
Phylogenetic Position ◽  
Crown Group ◽  
Brittle Stars ◽  
Major Clade ◽  
Early Ordovician ◽  
Stem Group ◽  
Successful Group

AbstractThe somasteroids are Ordovician star-shaped animals widely regarded as ancestors of Asterozoa, the group of extant echinoderms that includes brittle stars and starfish. The phylogenetic position of somasteroids makes them critical for understanding the origin and early evolution of crown group Echinodermata. However, the early evolution of asterozoans, the origin of their distinctive body organization and their relationships with other Cambrian and Ordovician echinoderms, such as edrioasteroids, blastozoans, crinoids, and other asterozoans, remain problematic due to the difficulties of comparing the calcitic endoskeleton of these disparate groups. Here we describe the new somasteroidCantabrigiaster fezouataensisfrom the Early Ordovician (Tremadocian) Fezouata Lagerstätte in Morocco.Cantabrigiastershares with other somasteroids the presence of rod-like virgal ossicles that articulate with the ambulacrals, but differs from all other known asterozoans in the absence of adambulacral ossicles defining the arm margins. The unique arm construction evokes parallels with non-asterozoan echinoderms. Developmentally informed Bayesian and parsimony based phylogenetic analyses, which reflect the homology of the biserial ambulacral ossicles in Paleozoic echinoderms according to the Extraxial-Axial Theory, recoverCantabrigiasteras basal within stem group Asterozoa. Our results indicate thatCantabrigiasteris the earliest diverging stem group asterozoan, revealing the ancestral morphology of this major clade and clarifying the affinities of problematic Ordovician taxa. Somasteroids are resolved as a paraphyletic grade within stem and crown group Asterozoa (starfishes), whereas stenuroids are paraphyletic within stem group Ophiuroidea (brittle stars).Cantabrigiasteralso illuminates the relationship between Ordovician crown group Echinodermata and its Cambrian stem lineage, which includes sessile forms with incipient radial symmetry such as edrioasteroids and blastozoans. The contentious Pelmatozoa hypothesis (i.e. monophyly of blastozoans and crinoids) is not supported; instead, blastozoans represent the most likely sister-taxon of crown group Echinodermata.Author summaryStarfish and brittle stars, collectively known as asterozoans, constitute a diverse and ecologically successful group of echinoderms that first appear in the fossil record some 480Ma. However, the early evolution of asterozoans, the origin of their distinctive body organization, and their phylogenetic relationships with Cambrian echinoderms remain largely unresolved. We describeCantabrigiaster fezouataensisgen. et sp. nov., a primitive asterozoan from the Fezouata Lagerstätte, Morocco, with a unique endoskeletal arm organization that reveals the ancestral morphology of this major clade. Bayesian and parsimony based phylogenetic analyses indicate thatCantabrigiasteris the earliest diverging stem group asterozoan, and resolve the phylogenetic position of Ordovician asterozoans such as somasteroids. Our analyses clarify the origin of crown group echinoderms relative to their problematic Cambrian stem group representatives.

scholarly journals A new somasteroid from the Fezouata Lagerstätte in Morocco and the Early Ordovician origin of Asterozoa

2021 ◽  
Vol 17 (1) ◽  
pp. 20200809
Author(s):  
Aaron W. Hunter ◽  
Javier Ortega-Hernández
Keyword(s):  
Bayesian Inference ◽  
Phylogenetic Analyses ◽  
Evolutionary Relationships ◽  
Crown Group ◽  
Brittle Stars ◽  
Early Ordovician ◽  
Stem Group ◽  
Lower Palaeozoic

The somasteroids are Lower Palaeozoic star-shaped animals widely regarded as ancestors of Asterozoa, the group of echinoderms that includes brittle stars and starfish. However, the origin of asterozoans, the assembly of their distinctive body organization, and their relationships with other Cambrian and Ordovician echinoderms remain problematic owing to the difficulties of comparing the endoskeleton between disparate groups. Here, we describe the new somasteroid Cantabrigiaster fezouataensis , a primitive asterozoan from the Early Ordovician Fezouata Lagerstätte in Morocco. Cantabrigiaster shares with other somasteroids a unique endoskeletal arm organization and the presence of rod-like virgal ossicles that articulate with the ambulacrals, but differs from all other known asterozoans in the absence of adambulacral ossicles defining the arm margins, evoking parallels with non-asterozoan echinoderms. Developmentally informed Bayesian and parsimony phylogenetic analyses, which reflect the homology of the biserial ambulacral ossicles in Palaeozoic echinoderms according to the extraxial–axial theory, recover Cantabrigiaster as the earliest divergent stem-group asterozoan. Our results illuminate the ancestral morphology of Asterozoa, and clarify the affinities of problematic Ordovician Asterozoa. Bayesian inference and parsimony demonstrate that somasteroids represent a paraphyletic grade within stem- and crown-group Asterozoa, whereas stenuroids are paraphyletic within stem-group Ophiuroidea. Our results also offer potential insights on the evolutionary relationships between asterozoans, crinoids and potential Cambrian stem-group representatives.

scholarly journals A Silurian short-great-appendage arthropod

2014 ◽  
Vol 281 (1778) ◽  
pp. 20132986 ◽  
Author(s):  
Derek J. Siveter ◽  
Derek E. G. Briggs ◽  
David J. Siveter ◽  
Mark D. Sutton ◽  
David Legg ◽  
...  
Keyword(s):  
Early Evolution ◽  
Phylogenetic Position ◽  
Crown Group ◽  
Hunsrück Slate ◽  
Stem Group ◽  
Sister Clade ◽  
The Uk ◽  
First Time

A new arthropod, Enalikter aphson gen. et sp. nov., is described from the Silurian (Wenlock Series) Herefordshire Lagerstätte of the UK. It belongs to the Megacheira (=short-great-appendage group), which is recognized here, for the first time, in strata younger than mid-Cambrian age. Discovery of this new Silurian taxon allows us to identify a Devonian megacheiran representative, Bundenbachiellus giganteus from the Hunsrück Slate of Germany. The phylogenetic position of megacheirans is controversial: they have been interpreted as stem chelicerates, or stem euarthropods, but when Enalikter and Bundenbachiellus are added to the most comprehensive morphological database available, a stem euarthropod position is supported. Enalikter represents the only fully three-dimensionally preserved stem-group euarthropod, it falls in the sister clade to the crown-group euarthropods, and it provides new insights surrounding the origin and early evolution of the euarthropods. Recognition of Enalikter and Bundenbachiellus as megacheirans indicates that this major arthropod group survived for nearly 100 Myr beyond the mid-Cambrian.

scholarly journals Evolution and palaeoecology of early turtles: a review based on recent discoveries in the Middle Jurassic

2017 ◽  
Author(s):  
Jérémy Anquetin
Keyword(s):  
New Species ◽  
Middle Jurassic ◽  
Phylogenetic Analyses ◽  
Morphological Characters ◽  
Phylogenetic Position ◽  
Crown Group ◽  
Stem Group ◽  
Three New Species ◽  
First Time ◽  
Lines Of Evidence

In recent years, no less than five new species of stem-group turtles have been described worldwide. Among them are three new turtles from Middle Jurassic deposits that partially fill a previous temporal and morphological gap in our knowledge of the early evolution of these shelled amniotes: Heckerochelys romani, Condorchelys antiqua and Eileanchelys waldmani. For the first time, the phylogenetic position of these three new species is tested in the context of the two presently competing cladistic models of turtle evolution. The addition of these taxa to each matrix does not favour or alter any of the two opposed hypotheses. However, it is demonstrated here that, by documenting yet unknown stages in the evolution of several morphological structures, these three species give stronger support to the model of an extended phylogenetic stem for turtles. These new lines of evidence include the structure of the vomer, the position of the aditus canalis stapedio-temporalis and of the posterior opening of the canalis cavernosus, and the morphology of the processus interfenestralis of the opisthotic. These characters should be considered for future phylogenetic analyses of turtle interrelationships.Recent discoveries also reinvigorate the debate about the palaeoecology of early turtles. Whereas simple morphological characters (e.g., shell fontanelle, ligamentous bridge, flattened carapace) can be misleading, forelimb proportions and shell bone histology have led to the conclusion that most stem turtles (i.e., Proganochelys quenstedti, Palaeochersis talampayensis, Proterochersis robusta, Kayentachelys aprix and meiolaniids) were terrestrial forms. On the contrary, it is generally accepted that crown-group turtles are ancestrally aquatic. Among the five recently described stem-group turtles, Odontochelys semitestacea and Eileanchelys waldmani have been convincingly interpreted as having aquatic habits, which suggests that basal turtles were ecologically diverse. More investigation is needed, but this will undoubtedly trigger further debate on the primitive ecology of turtles and on the origin of aquatic habits in Testudines (i.e., the crown-group), respectively.

scholarly journals Ancestral morphology of Ecdysozoa constrained by an early Cambrian stem group ecdysozoan

2020 ◽  
Vol 20 (1) ◽  
Author(s):  
Richard J. Howard ◽  
Gregory D. Edgecombe ◽  
Xiaomei Shi ◽  
Xianguang Hou ◽  
Xiaoya Ma
Keyword(s):  
Feeding Strategy ◽  
Phylogenetic Analyses ◽  
Morphological Diversity ◽  
Early Cambrian ◽  
Ancestral State ◽  
Crown Group ◽  
Stem Group ◽  
Chengjiang Biota ◽  
Fossil Records ◽  
Experimental Decay

Abstract Background Ecdysozoa are the moulting protostomes, including arthropods, tardigrades, and nematodes. Both the molecular and fossil records indicate that Ecdysozoa is an ancient group originating in the terminal Proterozoic, and exceptional fossil biotas show their dominance and diversity at the beginning of the Phanerozoic. However, the nature of the ecdysozoan common ancestor has been difficult to ascertain due to the extreme morphological diversity of extant Ecdysozoa, and the lack of early diverging taxa in ancient fossil biotas. Results Here we re-describe Acosmia maotiania from the early Cambrian Chengjiang Biota of Yunnan Province, China and assign it to stem group Ecdysozoa. Acosmia features a two-part body, with an anterior proboscis bearing a terminal mouth and muscular pharynx, and a posterior annulated trunk with a through gut. Morphological phylogenetic analyses of the protostomes using parsimony, maximum likelihood and Bayesian inference, with coding informed by published experimental decay studies, each placed Acosmia as sister taxon to Cycloneuralia + Panarthropoda—i.e. stem group Ecdysozoa. Ancestral state probabilities were calculated for key ecdysozoan nodes, in order to test characters inferred from fossils to be ancestral for Ecdysozoa. Results support an ancestor of crown group ecdysozoans sharing an annulated vermiform body with a terminal mouth like Acosmia, but also possessing the pharyngeal armature and circumoral structures characteristic of Cambrian cycloneuralians and lobopodians. Conclusions Acosmia is the first taxon placed in the ecdysozoan stem group and provides a constraint to test hypotheses on the early evolution of Ecdysozoa. Our study suggests acquisition of pharyngeal armature, and therefore a change in feeding strategy (e.g. predation), may have characterised the origin and radiation of crown group ecdysozoans from Acosmia-like ancestors.

New insights into the evolution of lateral compound eyes in Palaeozoic horseshoe crabs

2019 ◽  
Vol 187 (4) ◽  
pp. 1061-1077 ◽  
Author(s):  
Russell D C Bicknell ◽  
Lisa Amati ◽  
Javier Ortega-Hernández
Keyword(s):  
Convincing Evidence ◽  
Limulus Polyphemus ◽  
Phylogenetic Position ◽  
Compound Eyes ◽  
Crown Group ◽  
Widespread Occurrence ◽  
Eye Evolution ◽  
Stem Group ◽  
Lateral Eye ◽  
Horseshoe Crabs

Abstract Vision allows animals to interact with their environment. Aquatic chelicerates dominate the early record of lateral compound eyes among non-biomineralizing crown-group euarthropods. Although the conservative morphology of lateral eyes in Xiphosura is potentially plesiomorphic for Euarthropoda, synziphosurine eye organization has received little attention despite their early diverging phylogenetic position. Here, we re-evaluate the fossil evidence for lateral compound eyes in the synziphosurines Bunodes sp., Cyamocephalus loganensis, Legrandella lombardii, Limuloides limuloides, Pseudoniscus clarkei, Pseudoniscus falcatus and Pseudoniscus roosevelti. We compare these data with lateral eyes in the euchelicerates Houia yueya, Kasibelinurus amicorum and Lunataspis aurora. We find no convincing evidence for lateral eyes in most studied taxa, and Pseudoniscus roosevelti and Legrandella lombardii are the only synziphosurines with this feature. Our findings support two scenarios for euchelicerate lateral eye evolution. The elongate-crescentic lateral eyes of Legrandella lombardii might represent the ancestral organization, as suggested by the phylogenetic position of this taxon in stem-group Euchelicerata. Alternatively, the widespread occurrence of kidney-shaped lateral eyes in stem-group Xiphosura and stem-group Arachnida could represent the plesiomorphic condition; Legrandella lombardii eyes would therefore be derived. Both evolutionary scenarios support the interpretation that kidney-shaped lateral eyes are ancestral for crown-group Euchelicerata and morphologically conserved in extant Limulus polyphemus.

scholarly journals Jaw elements in Plumulites bengtsoni confirm that machaeridians are extinct armoured scaleworms

2019 ◽  
Vol 286 (1907) ◽  
pp. 20191247 ◽  
Author(s):  
Luke A. Parry ◽  
Gregory D. Edgecombe ◽  
Dan Sykes ◽  
Jakob Vinther
Keyword(s):  
Phylogenetic Analysis ◽  
Early Evolution ◽  
The Body ◽  
Common Origin ◽  
Discrete Elements ◽  
Crown Group ◽  
Total Group ◽  
Early Ordovician ◽  
Jaw Apparatus ◽  
Competing Hypotheses

Machaeridians are Palaeozoic animals that are dorsally armoured with serialized, imbricating shell plates that cover or enclose the body. Prior to the discovery of an articulated plumulitid machaeridian from the Early Ordovician of Morocco that preserved unambiguous annelid characters (segmental parapodia with chaetae), machaeridians were a palaeontological mystery, having been previously linked to echinoderms, barnacles, tommotiids (putative stem-group brachiopods) or molluscs. Although the annelid affinities of machaeridians are now firmly established, their position within the phylum and relevance for understanding the early evolution of Annelida is less secure, with competing hypotheses placing Machaeridia in the stem or deeply nested within the crown group of annelids. We describe a scleritome of Plumulites bengtsoni from the Fezouata Formation of Morocco that preserves an anterior jaw apparatus consisting of at least two discrete elements that exhibit growth lines. Although jaws have multiple independent origins within the annelid crown group, comparable jaws are present only within Phyllodocida, the clade that contains modern aphroditiforms (scaleworms and relatives). Phylogenetic analysis places a monophyletic Machaeridia within the crown group of Phyllodocida in total-group Aphroditiformia, consistent with a common origin of machaeridian shell plates and scaleworm elytrae. The inclusion of machaeridians in Aphroditiformia truncates the ghost lineage of Phyllodocida by almost a hundred million years.

First plesiosaur remains from the lower Cretaceous of the Neuquén Basin, Argentina

2003 ◽  
Vol 77 (4) ◽  
pp. 784-789 ◽  
Author(s):  
Dario G. Lazo ◽  
Marcela Cichowolski
Keyword(s):  
Lower Cretaceous ◽  
Sister Group ◽  
Monophyletic Group ◽  
Phylogenetic Position ◽  
Crown Group ◽  
Marine Reptiles ◽  
Stem Group ◽  
Neuquen Basin ◽  
Neuquén Basin

Plesiosaurs constitute a monophyletic group whose stratigraphical range is uppermost Triassic to uppermost Cretaceous (Brown, 1981). They were large predatory marine reptiles, highly adapted for submarine locomotion, with powerful paddle-like limbs and heavily reinforced limb girdles (Saint-Seine, 1955; Romer, 1966; Carroll, 1988; Benton, 1990). The Plesiosauria clade belongs to the Sauropterygia, which has recently been hypothesized as the sister-group of the Ichthyosauria. Together with that clade they form the Euryapsida (Caldwell, 1997). The Sauropterygia can be subdivided into relatively plesiomorphic stem-group taxa from the Triassic (Placodonts, Nothosauroids, and Pistosauroids), and the obligatorily marine crown-group Plesiosauria (Rieppel, 1999). Plesiosaurs are traditionally divided into two superfamilies: Plesiosauroidea, with usually small heads and long necks; and Pliosauroidea, with larger heads and shorter necks (Welles, 1943; Persson, 1963; Brown, 1981). Plesiosauroidea contains three families: Plesiosauridae, Cryptoclididae, and Elasmosauridae (Brown, 1981; Brown and Cruickshank, 1994). The validity of the Polycotylidae Cope, 1869, has long been questioned and its phylogenetic position among Plesiosauria debated, as many consider it to be related to the Pliosauridae or to be a sister-group of the Elasmosauridae (Sato and Storrs, 2000; O'Keefe, 2001).

scholarly journals Evolution and palaeoecology of early turtles: a review based on recent discoveries in the Middle Jurassic

2011 ◽  
Vol 182 (3) ◽  
pp. 231-240 ◽  
Author(s):  
Jérémy Anquetin
Keyword(s):  
New Species ◽  
Middle Jurassic ◽  
Morphological Characters ◽  
Early Evolution ◽  
Bone Histology ◽  
Phylogenetic Position ◽  
Crown Group ◽  
Three New Species ◽  
First Time ◽  
Lines Of Evidence

AbstractIn recent years, no less than five new species of early turtles have been described worldwide. Among them are three new turtles from Middle Jurassic deposits that partially fill a previous temporal and morphological gap in our knowledge of the early evolution of these shelled amniotes: Heckerochelys romani, Condorchelys antiqua and Eileanchelys waldmani. For the first time, the phylogenetic position of these three new species is tested in the context of the two presently competing cladistic models of turtle evolution. The addition of these taxa to each matrix does not favour or alter any of the two opposed hypotheses. However, it is demonstrated here that, by documenting yet unknown stages in the evolution of several morphological structures, these three species give stronger support to the model of an extended phylogenetic stem for turtles. These new lines of evidence include the structure of the vomer, the position of the aditus canalis stapedio-temporalis and of the posterior opening of the canalis cavernosus, and the morphology of the processus interfenestralis of the opisthotic.Recent discoveries also reinvigorate the debate about the palaeoecology of early turtles. Whereas simple morphological characters (e.g., shell fontanelle, ligamentous bridge, flattened carapace) can be misleading, forelimb proportions and shell bone histology have led to the conclusion that most stem turtles (i.e., Proganochelys quenstedti, Palaeochersis talampayensis, Proterochersis robusta, Kayentachelys aprix and meiolaniids) were terrestrial forms. At least two out of the five recently described early turtles have been convincingly interpreted as having aquatic habits: Odontochelys semitestacea and Eileanchelys waldmani. More investigation is needed, but this will undoubtedly trigger further debate on the primitive ecology of turtles and on the origin of aquatic habits in testudines (i.e., the crown-group), respectively.

scholarly journals Large neotheropods from the Upper Triassic of North America and the early evolution of large theropod body sizes

2019 ◽  
Vol 93 (5) ◽  
pp. 1010-1030 ◽  
Author(s):  
Christopher T. Griffin
Keyword(s):  
Body Size ◽  
Phylogenetic Analyses ◽  
Large Body ◽  
Upper Triassic ◽  
Early Evolution ◽  
Phylogenetic Position ◽  
Ancestral State ◽  
Femur Length ◽  
Theropod Dinosaurs ◽  
Body Sizes

AbstractLarge body sizes among nonavian theropod dinosaurs is a major feature in the evolution of this clade, with theropods reaching greater sizes than any other terrestrial carnivores. However, the early evolution of large body sizes among theropods is obscured by an incomplete fossil record, with the largest Triassic theropods represented by only a few individuals of uncertain ontogenetic stage. Here I describe two neotheropod specimens from the Upper Triassic Bull Canyon Formation of New Mexico and place them in a broader comparative context of early theropod anatomy. These specimens possess morphologies indicative of ontogenetic immaturity (e.g., absence of femoral bone scars, lack of co-ossification between the astragalus and calcaneum), and phylogenetic analyses recover these specimens as early-diverging neotheropods in a polytomy with other early neotheropods at the base of the clade. Ancestral state reconstruction for body size suggests that the ancestral theropod condition was small (~240 mm femur length), but the ancestral neotheropod was larger (~300–340 mm femur length), with coelophysoids experiencing secondary body size reduction, although this is highly dependent on the phylogenetic position of a few key taxa. Theropods evolved large body sizes before the Triassic–Jurassic extinction, as hypothesized in most other ancestral state reconstructions of theropod body sizes, but remained rare relative to smaller theropods until the Jurassic.

scholarly journals 40 new specimens of Ichthyornis provide unprecedented insight into the postcranial morphology of crownward stem group birds.

2022 ◽  
Author(s):  
Juan Benito ◽  
Albert Chen ◽  
Laura E. Wilson ◽  
Bhart-Anjan S. Bhullar ◽  
David Burnham ◽  
...  
Keyword(s):  
Morphological Evolution ◽  
Phylogenetic Analyses ◽  
Three Dimensional ◽  
Ontogenetic Changes ◽  
Crown Group ◽  
Postcranial Morphology ◽  
Stem Group ◽  
New Information ◽  
The 19Th Century ◽  
Future Work

Ichthyornis has long been recognized as a pivotally important fossil taxon for understanding the latest stages of the dinosaur-bird transition, but little significant new postcranial material has been brought to light since initial descriptions of partial skeletons in the 19th Century. Here, we present new information on the postcranial morphology of Ichthyornis from 40 previously undescribed specimens, providing the most detailed morphological assessment of Ichthyornis to date. The new material includes four partially complete skeletons and numerous well-preserved isolated elements, enabling new anatomical observations such as muscle attachments previously undescribed for Mesozoic euornitheans. Among the elements that were previously unknown or poorly represented for Ichthyornis, the new specimens include an almost-complete axial series, a hypocleideum-bearing furcula, radial carpal bones, fibulae, a complete tarsometatarsus bearing a rudimentary hypotarsus, and one of the first-known nearly complete three-dimensional sterna from a Mesozoic avialan. Several pedal phalanges are preserved, revealing a remarkably enlarged pes presumably related to foot-propelled swimming. Although diagnosable as Ichthyornis, the new specimens exhibit a substantial degree of morphological variation, some of which may relate to ontogenetic changes. Phylogenetic analyses incorporating our new data and employing alternative morphological datasets recover Ichthyornis stemward of Hesperornithes and Iaceornis, in line with some recent hypotheses regarding the topology of the crownward-most portion of the avian stem group, and we establish phylogenetically-defined clade names for relevant avialan subclades to help facilitate consistent discourse in future work. The new information provided by these specimens improves our understanding of morphological evolution among the crownward-most non-neornithine avialans immediately preceding the origin of crown group birds.

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