New chelicerate (= spiders + scorpions) ancestor described
Another dip into invertebrates
prompted by the discovery of tiny (5mm-1cm) Early Ordovician Setapedites (Figs 1, 2).
Lustri et al 2024 wrote,
“Little is known regarding the origin and early evolution of the euchelicerate body plan.”
Actually we looked at the origin of spiders and scorpions two years ago (Fig 2) here modified with the addition of Setpedites.
Figure 1. Modified from Lustri et al 2024, Setapedites in their cladogram.
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/05/chelicerates-lustri588.jpg?w=241″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/05/chelicerates-lustri588.jpg?w=584″ class=”size-full wp-image-86014″ src=”https://pterosaurheresies.files.wordpress.com/2024/05/chelicerates-lustri588.jpg?w=584&h=727″ alt=”Figure 1. Modified from Lustri et al 2024, Setapedites in their cladogram.” width=”584″ height=”727″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/05/chelicerates-lustri588.jpg?w=584&h=727 584w, https://pterosaurheresies.files.wordpress.com/2024/05/chelicerates-lustri588.jpg?w=120&h=150 120w, https://pterosaurheresies.files.wordpress.com/2024/05/chelicerates-lustri588.jpg?w=241&h=300 241w, https://pterosaurheresies.files.wordpress.com/2024/05/chelicerates-lustri588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Modified from Lustri et al 2024, Setapedites in their cladogram. Dorsal and ventral shapes split each taxon in half lengthwise. Note the absence of traditional trilobites, spiders and scorpions here.
From phys.org publicity online:
“This question has puzzled paleontologists ever since the study of ancient fossils began. It was impossible to identify with certainty any forms among early arthropods that shared enough similarities with modern species to be considered ancestors.”
“Lorenzo Lustri, then a Ph.D. student at the University of Lausanne (UNIL)’s Faculty of Geosciences and Environment, provided the missing piece of the puzzle.”
“Fossils from the Fezouata Shale were discovered in the early 2000s and have undergone extensive analysis. However, the fossil illustrated in the publication, one of the most abundant in the deposit, had never been described before. Measuring between 5 and 10 millimeters in size, it has been named Setapedites abundantis. This animal makes it possible, for the first time, to trace the entire lineage of chelicerates, from the appearance of the earliest arthropods to modern spiders, scorpions and horseshoe crabs.”
The authors make no mention of the word ‘trilobite’ (Fig 2) in their text.
‘This question’ was answered two years ago here with a wider gamut of example taxa. Additional transitional taxa, like Setipedites (Figs 1, 2) are always expected to appear over time and are always welcome to make morphological gaps smaller.
Figure 2. Sedipedites added to a 2022 chart portraying spider and scorpion evolution from a trilobite ancestor with a telson (red posterior spine). Note the gradual reduction of the head and thorax plates along with the gradual enlargement of the oral and locomotion appendages. The telson (red posterior) is always retained.
” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/05/spider_evolution588-1.jpg?w=38″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/05/spider_evolution588-1.jpg?w=128″ class=”size-full wp-image-86020″ src=”https://pterosaurheresies.files.wordpress.com/2024/05/spider_evolution588-1.jpg?w=584&h=4671″ alt=”Figure 2. Sedipedites added to a 2022 chart portraying spider and scorpion evolution from a trilobite ancestor with a telson (red posterior spine). Note the gradual reduction of the head and thorax plates along with the gradual enlargement of the oral and locomotion appendages. The telson (red posterior) is always retained.” width=”584″ height=”4671″ srcset=”https://pterosaurheresies.files.wordpress.com/2024/05/spider_evolution588-1.jpg?w=584&h=4671 584w, https://pterosaurheresies.files.wordpress.com/2024/05/spider_evolution588-1.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2. Sedipedites added to a 2022 chart portraying spider and scorpion evolution from a trilobite ancestor with a telson (red posterior spine). Note the gradual reduction of the head and thorax plates along with the gradual enlargement of the oral and locomotion appendages. The telson (red posterior) is always retained.
You’ll note the appearance of
the oral and locomotion appendages slightly beyond the rim of the carapace in dorsal view in Setipedites (Fig 2) as it links extinct trilobites with extant trilobites = spiders and scorpions, in which the oral and locomotion appendages are much larger relative to the carapace and thorax segments. Horseshoe crabs, like Limulus (Figs 1, 2), retain ancient proportions. If you support the concept of monophyly, this is evolution in action.
The telson is always retained. If you know of any exceptions within this clade, please send along examples of its absence.
References
Lustri L, Gueriau P and Daley AC 2024. Lower Ordovician synziphosurine reveals early euchelicerate diversity and evolution, Nature Communications.
DOI: 10.1038/s41467-024-48013-w
wiki/Sedipedites – not yet posted
Publicity
phys.org/news/2024-05-discovery-ancestors-scorpions-spiders-horseshoe
twitter.com/LorenzoLustri/status/1787848476245504405
eurekalert.org/news-releases/1044566
sci.news/paleontology/setapedites-abundantis-12931.html
Source: https://pterosaurheresies.wordpress.com/2024/05/16/new-chelicerate-spiders-scorpions-ancestor-described/
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