The dynamics of stem and crown groups

Budd, Graham E.; Mann, Richard P.

doi:10.1101/633008

Cited by 17 publications

(27 citation statements)

References 32 publications

(31 reference statements)

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“…The gastropod Pelagiella atlantoides and apparent sabellid Pseudopelagiella exigua prompt consideration of what comprises stem group and crown group in the CER. Budd & Mann (2020) detailed that: (1) stem and crown are divisions of the total group clade; (2) preservational (and discovery) issues affect understanding of clade development; and (3) misidentification of stem group taxa takes place. To this might be added another consideration: that phylogenies, however sophisticated, are interpretations of relationships that reflect the current taxonomy and known chronostratigraphy of biotic groups, with taxonomy often reflecting the consensus of a few specialists.…”

Section: Discussionmentioning

confidence: 99%

Lophotrochozoa in the Cambrian evolutionary radiation and the Pelagiella problem

Landing

Geyer

Jirkov

et al. 2021

Papers in Palaeontology

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Tiny snail-like conchs of Pelagiella Matthew are important in discussions of lophotrochozoan and mollusc origins in the Cambrian evolutionary radiation (CER). The limited morphological features of Pelagiella conchs have led to the 'Pelagiella problem': (1) poorly distinguished Pelagiella species; (2) an exceptional genus range across all Cambrian palaeocontinents and climate belts; (3) an unparalleled genus-lifespan (c. 31 myr) in the early Palaeozoic; and (4)the assumption that all Pelagiella spp. are gastropods, which compromises lophotrochozoan and mollusc phylogenies. The type species P. atlantoides is a gastropod, but we show that so-called Pelagiella species with sub-to-triangular apertures and turbiniform conchs are not referable to the genus. Reevaluation of spectacular specimens of Pelagiella exigua Resser & Howell show it to be a polychaete with two apertural paleal chaetae fan arrays. Given that some modern sedentary polychaetes have paleal fan arrays, P. exigua is regarded as a likely member of the Sabellida Levinsen and is assigned to Pseudopelagiella gen. nov. Other Pelagiella spp. and forms with similar conchs are probably polychaetes not relevant to syntheses of early mollusc diversification. Pelagiellidae fam. nov. should be restricted to the late early Cambrian (c. 508 Ma) P. atlantoides. Pseudopelagiella exigua was unattached, probably benthic and lived with the apical conch surface on the sea floor, a mode of life similar to that of another family of the Sabellida, the sessile spirorbids that are cemented to substrate. Among numerous crown groups, the late early Cambrian eve of the CER included the polychaete Pseudopelagiella exigua and gastropod P. atlantoides.

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Section: Discussionmentioning

confidence: 99%

Lophotrochozoa in the Cambrian evolutionary radiation and the Pelagiella problem

Landing

Geyer

Jirkov

et al. 2021

Papers in Palaeontology

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“…It is not easy to persist at low diversity for hundreds of millions of years; there is just too much risk of going extinct. These phylogenetic models also indicate that after the crown group emerges, the stem group rapidly declines [38]. This again reflects survivorship bias; if they did not decline rapidly, it is unlikely they would have gone extinct-which by definition, they must have, given that they are stem groups.…”

Section: Models Of Early Eukaryote Evolution and The Age Of Last Eukamentioning

confidence: 92%

“…These results do not hold in special cases, e.g. when the crown diversification follows a mass extinction [38], and, therefore, it is not easy to know how to apply them to the particular case of eukaryotes. However, there is no obvious evidence for mass extinction in the Mesoproterozoic or early Tonian.…”

Section: Models Of Early Eukaryote Evolution and The Age Of Last Eukamentioning

confidence: 98%

Insights into eukaryogenesis from the fossil record

Porter

2020

Interface Focus.

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Eukaryogenesis—the process by which the eukaryotic cell emerged—has long puzzled scientists. It has been assumed that the fossil record has little to say about this process, in part because important characters such as the nucleus and mitochondria are rarely preserved, and in part because the prevailing model of early eukaryotes implies that eukaryogenesis occurred before the appearance of the first eukaryotes recognized in the fossil record. Here, I propose a different scenario for early eukaryote evolution than is widely assumed. Rather than crown group eukaryotes originating in the late Paleoproterozoic and remaining ecologically minor components for more than half a billion years in a prokaryote-dominated world, I argue for a late Mesoproterozoic origin of the eukaryotic crown group, implying that eukaryogenesis can be studied using the fossil record. I review the proxy records of four crown group characters: the capacity to form cysts as evidenced by the presence of excystment structures; a complex cytoskeleton as evidenced by spines or pylomes; sterol synthesis as evidenced by steranes; and aerobic respiration—and therefore mitochondria—as evidenced by eukaryotes living in oxic environments, and argue that it might be possible to use these proxy records to infer the order in which these characters evolved. The records indicate that both cyst formation and a complex cytoskeleton appeared by late Paleoproterozoic time, and sterol synthesis appeared in the late Mesoproterozioc or early Neoproterozoic. The origin of aerobic respiration cannot as easily be pinned down, but current evidence permits the possibility that it evolved sometime in the Mesoproterozoic.

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“…These distinctions are absolutely critical for the question we address here; for more detailed explanations, we refer the reader to previous reviews of these definitions (Magallón, 2004;Doyle, 2012;Marshall, 2019;Budd and Mann, 2020). Interestingly, Cantino et al (2007) proposed three distinct names for the three clades associated with these three ages, Pan-Angiospermae (the total clade of angiosperms, including its crown group and all of its stem relatives), Apo-Angiospermae (the clade of all living and fossil angiosperms possessing a closed carpel), and Angiospermae (the crown clade of angiosperms).…”

Section: Angiosperms Have Three Agesmentioning

confidence: 99%

“…In particular, further exploring of heterogeneity in rates of diversification, fossil preservation, and morphological evolution in the early history of angiosperms (including before and after the origin of the crown node) may bring important future clues to explain the long stem branch subtending crown angiosperms and, hopefully, better quantify the likelihood of various scenarios. For instance, Budd and Mann (2020) recently quantified the macroevolutionary dynamics of stem and crown groups using extensive simulations, including perturbations such as mass extinctions. Their results provide a plausible explanation for the contrast between the long stem branch leading to crown angiosperms and the comparatively much shorter stem lineage of acrogymnosperms, and predict low diversity throughout most of the angiosperm stem lineage.…”

Section: Moving Forwardmentioning

confidence: 99%

The age of flowering plants is unknown

Sauquet¹,

Ramírez‐Barahona²,

Magallón³

2021

Preprint

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The origin of flowering plants (angiosperms) was one of the most transformative events in the history of our planet. Despite considerable interest from multiple research fields, numerous questions remain, including the age of the group as a whole. Recent studies have reported a perplexing range of estimates for the crown-group age of angiosperms, from ca. 140 Ma (Early Cretaceous) to 270 Ma (Permian). Both ends of the spectrum are now supported by both quantitative analyses of the fossil record and fossil-calibrated molecular dating analyses. Here, we first clarify and distinguish among the three ages of angiosperms: the age of their divergence with acrogymnosperms (stem age), the age(s) of emergence of their unique, distinctive features including flowers (morphological age), and the age of the most recent common ancestor of all their living species (crown age). We then demonstrate, based on recent studies, that fossil-calibrated molecular dating estimates of the crown-group age of angiosperms have little to do with either the amount of molecular data or the number of internal fossil calibrations included. Instead, we argue that this age is almost entirely conditioned by its own prior. Lastly, we discuss which future discoveries or novel types of analyses are most likely to bring more definitive answers. In the meantime, we propose that the age of angiosperms is best described as unknown (140–270 Ma) and that future work that depends on the time scale of flowering plant diversification be designed to integrate over this vexing uncertainty.

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The dynamics of stem and crown groups

Cited by 17 publications

References 32 publications

Lophotrochozoa in the Cambrian evolutionary radiation and the Pelagiella problem

Lophotrochozoa in the Cambrian evolutionary radiation and the Pelagiella problem

Insights into eukaryogenesis from the fossil record

The age of flowering plants is unknown

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