Librostoma

{{Automatic taxobox

| image = Asaphus.jpg

| image_caption = Asaphus lepidurus

| fossil_range = {{Fossil range|Cambrian series 2|Lopingian}}

| taxon = Librostoma

| authority = Fortey, 1990Ontogeny, hypostome attachment and trilobite classification. RA Fortey, Palaeontology, 1990

| subdivision_ranks = Orders

| subdivision =

{{Extinct}}Asaphida
{{Extinct}}Harpetida
{{Extinct}}Phacopida?{{Efn|Two primitive calymenines, (Pharostomina and Bavarilla), had natant hypostomes, suggesting that the Cambrian sister taxon is likely also natant, making them Librostomes.}}
{{Extinct}}Proetida
{{Extinct}}Ptychopariida
{{Extinct}}Trinucleida?(proposed){{cite journal |last1=Bignon |first1=Arnaud |last2=Waisfield |first2=Beatriz G. |last3=Vaccari |first3=Emilio |last4=Chatterton |first4=Brian D. E. |title=Reassessment of the Order Trinucleida (Trilobita) |date=2020 |url=https://www.tandfonline.com/doi/abs/10.1080/14772019.2020.1720324 |journal=Journal of Systematic Palaeontology |volume=18 |issue=13 |pages=1061–1077 |doi=10.1080/14772019.2020.1720324|bibcode=2020JSPal..18.1061B |s2cid=212995185 |url-access=subscription }}

}}

Librostoma is a subclass of trilobites defined by having a natant hypostome, which is a hypostome (mineralized trilobite mouthpart) that is free from the anterior doublure and aligned with the anterior of the glabella,{{cite web |title=The Hypostome |url=https://trilobites.info/hypoterms.htm}} this is unlike a conterminant hypostome, which is attached to the exoskeleton.{{cite web |last1=Whittington |first1=H. B. |title=Reflections on the Classification of the Trilobita |url=https://www.researchgate.net/publication/256082447 |publisher=University of Cambridge}}

They are a large and long lasting group, with some of the candidates for the first trilobites being librostomes in the order ptychopariida; and all post-Devonian{{Efn|Arguably all Post-Frasnian triobites as phacopids are cladistically included}} trilobites being in the order proetida, who are librostomes.

Some librostomes may not even be within the group, with Whittington (2003) placing some genera (Hemibarrandia, Lakaspis, Peraspis and Symphysurina) of the asaphid family Nileidae in the order Corynexochida.{{cite journal |last1=Whittington |first1=Harry B. |title=The trilobite family Nileidae: morphology and classification |date=2003 |journal=Palaeontology |volume=46 |issue=4 |pages=635–646 |doi=10.1111/1475-4983.00313|bibcode=2003Palgy..46..635W |doi-access=free }}

Taxonomy

File:Cummingella belisama dorsal light2 CRF.jpg|Cummingella belisama, a Phillipsiid Proetid from the Carboniferous.

File:Harpes perradiatus fossil trilobite (Devonian, Morocco).jpg|Harpes perridatus, a Harpetid from the Devonian of Morocco.

File:ElrathiaKingi.jpg| Elrathia kingii, a ptychopariid from the Middle cambrian

Librostomes make up a large component of the Trilobita. with Traditionally, four orders are placed in the librostoma; they are the: Asaphida, Harpetida, Proetida, and Ptychopariida; however, two primitive Phacopids (Pharostomina and Bavarilla) have natant hypostomes, indicating that the Phacopids are also librostomes.{{cite web |title=Phacopida fact sheet |url=https://trilobites.info/ordphacopida.htm |website=trilobites.info}} In more recent publications, the asaphid superfamily Trinucleioidea is placed in its own order, Trinucleida,{{cite journal |last1=Bignon |first1=Arnaud |last2=Waisfield |first2=Beatriz G. |last3=Vaccari |first3=Emilio |last4=Chatterton |first4=Brian D. E. |title=Reassessment of the Order Trinucleida (Trilobita) |date=2020 |url=https://www.tandfonline.com/doi/abs/10.1080/14772019.2020.1720324 |journal=Journal of Systematic Palaeontology |volume=18 |issue=13 |pages=1061–1077 |doi=10.1080/14772019.2020.1720324|bibcode=2020JSPal..18.1061B |s2cid=212995185 |url-access=subscription }}{{cite web |title=Trilobite order Trinucleioidea |url=https://trilobites.info/ordtrinucleida.htm |website=trilobites.info}} however, even today some studies recover them as asaphids.{{Cite journal |last=Beech |first=James D. |last2=Bottjer |first2=David J. |last3=Smith |first3=Nathan D. |date=2024-10-15 |title=Parallel evolution of unusual ‘harpiform’ morphologies in distantly related trilobites |url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/parallel-evolution-of-unusual-harpiform-morphologies-in-distantly-related-trilobites/4EE10129DFF4B79E9847A4FB19C1931A |journal=Journal of Paleontology |language=en |pages=1–12 |doi=10.1017/jpa.2024.47 |issn=0022-3360|doi-access=free }}

Evolution

=Origins=

The first librostomes were Ellipsocephaloid Ptychopariida in the family Bigotinidae from the early cambrian.{{cite web |title=The earliest trilobites |url=https://www.trilobites.info/firsttrilos.htm}} This group was once thought to be in the order redlichiida, but are now considered primitive Ptychopariida.

=Evolutionary history=

File:Asaphus kowalewskii 3.jpg|Asaphus kowalewskii from the Ordovician of Russia.

File:Opipeuterella swimming.png|Opipueterella is a genus of swimming trilobite in the family Telephinidae; order Proetida.

File:Ampyx cetsarum - specimen number MGL 96718 - white (cropped).jpg|Ampyx, An Ordovician Trinucleioid. these trilobites may have gone in single-file lines from deeper water to shallow water at night, mirroring modern Spiny lobsters.

During the Middle Cambrian, the Ptychopariida became the dominant trilobites. Around that time, the Asaphida and Trinucleida would first appear.

The first Harpetida would appear in the late cambrian with the primitive genera Baikadamaspis and Entomaspis of the family Entomaspididae.

The first Proetida appeared during the cambrian-ordovician transition. Since the earliest Phacopida (which are most likely librostomes) appeared during the early ordovician, it is assumed that a late cambrian sister taxon was present.

During the Ordovician, the Ptychopariida declined. After the decline of the Ptychopariida, the order Asaphida rose to prominence, becoming the dominant order of trilobites by the end of the mid-ordovician.

File:Triarthrus eatoni ventral cropped.jpg, one of the last ptychopariids.]]

The End-Ordovician extinction event would drastically reduce trilobite diversity, with librostomes included. The already endangered Ptychopariida would go extinct. All trilobites with planktonic or Nektonic life stages would go extinct,{{Cite journal |last1=Chatterton |first1=Brian D. E. |last2=Speyer |first2=Stephen E. |date=April 1989 |title=Larval ecology, life history strategies, and patterns of extinction and survivorship among Ordovician trilobites |url=https://www.cambridge.org/core/journals/paleobiology/article/abs/larval-ecology-life-history-strategies-and-patterns-of-extinction-and-survivorship-among-ordovician-trilobites/ABD0735C7F5312639FE775B71FA7D087 |journal=Paleobiology |language=en |volume=15 |issue=2 |pages=118–132 |doi=10.1017/S0094837300009313 |bibcode=1989Pbio...15..118C |issn=0094-8373|url-access=subscription }} which included the swimming Proetida family Telephinidae, and all Asaphids & Trinucleids except for the raphiophorid Raphiophorus, which would go extinct shortly after in the Silurian.

Diversity

Librostomes occupied many niches, sizes, and ecologies. Both some of the largest (Ordovician asaphids Isotelus rex, and Hungioides) and smallest Trilobites (Ordovician ptychopariid Acanthopleurella stipulae) are librostomes.{{Citation |last=Forli |first=Maurizio |title=Scorpions, Ants, and Other Stone Insects. The Understanding of Trilobites Over the Centuries |date=2022 |work=The History of Fossils Over Centuries: From Folklore to Science |pages=417–442 |editor-last=Forli |editor-first=Maurizio |url=https://link.springer.com/chapter/10.1007/978-3-031-04687-2_23 |access-date=2024-10-01 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-031-04687-2_23 |isbn=978-3-031-04687-2 |last2=Guerrini |first2=Andrea |editor2-last=Guerrini |editor2-first=Andrea|url-access=subscription }} Many trilobites in the group had some ability to stay above the sea floor, such as asaphids in the superfamilies Cyclopygoidea and Remopleuroidea,{{Cite journal |last=Esteve |first=Jorge |last2=López-Pachón |first2=Matheo |date=2023-09-01 |title=Swimming and feeding in the Ordovician trilobite Microparia speciosa shed light on the early history of nektonic life habits |url=https://www.sciencedirect.com/science/article/pii/S0031018223003097 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=625 |pages=111691 |doi=10.1016/j.palaeo.2023.111691 |issn=0031-0182|doi-access=free }}{{Cite web |last=Shiino |first=Yuta |last2=Kuwazuru |first2=Osamu |last3=Suzuki |first3=Yutaro |last4=Ono |first4=Satoshi |last5=Masuda |first5=Chihiro |date=2014-06-01 |title=Pelagic or benthic? Mode of life of the remopleuridid trilobite Hypodicranotus striatulus. {{!}} EBSCOhost |url=https://openurl.ebsco.com/EPDB:gcd:1:9925153/detailv2?sid=ebsco:plink:scholar&id=ebsco:gcd:96213802&crl=c |access-date=2024-10-01 |website=openurl.ebsco.com |language=en}} proetids in the family Telephinidae, and perhaps some Cheirurid phacopids such as Deiphon and Crotalocephalus.

The Asaphida, Ptychopariida (and Phacopida if included) are some of the most morphologically and cladistically diverse orders of Trilobites. Meanwhile, the Proetida and Harpetida had a more modest morphological diversity; the former, sometimes referred to as "garden variety trilobites",{{Cite journal |last=Lieberman |first=Bruce S. |last2=Karim |first2=Talia S. |date=2010-03-01 |title=Tracing the trilobite tree from the root to the tips: A model marriage of fossils and phylogeny |url=https://www.sciencedirect.com/science/article/abs/pii/S1467803909000565 |journal=Arthropod Structure & Development |series=Fossil Record and Phylogeny of the Arthropoda |volume=39 |issue=2 |pages=111–123 |doi=10.1016/j.asd.2009.10.004 |issn=1467-8039|url-access=subscription }} containing the most diverse family of trilobites in terms of genera, and the latter being more modest in both terms of diversity.