Watsonella

{{Short description|Extinct genus of molluscs}} {{Automatic taxobox | fossil_range = Cambrian Stage 2, c. {{fossil range|530|525}} | taxon = Watsonella | authority = Grabau, 1900 | type_species = Watsonella crosbyi | type_species_authority = Grabau, 1900 | subdivision_ranks = Accepted species | subdivision = * ''Watsonella crosbyi'' Grabau, 1900 | synonyms = * ''Heraultia'' Cobbold, 1935 * ''Heraultipegma'' Pojeta and Runnegar, 1976 }}

'''''Watsonella''''' is an extinct genus of mollusc known from early (Terreneuvian) Cambrian strata. It has been hypothesized to be close to the origin of bivalves. It contains a single species, '''''Watsonella crosbyi'''''.

The genus is closely related to ''Anabarella'', with which it bears many morphological similarities, including a laminar internal shell microstructure said to connect it with the early bivalves ''Fordilla'' and ''Pojetaia''.

==Taxonomy== ''Watsonella'' was described by Amadeus William Grabau in 1900, with the type species ''Watsonella crosbyi''.<ref name="Grabau1900"/> The type specimen was found in a fossil-filled boulder collected by Thomas Augustus Watson, the assistant of Alexander Graham Bell who received the first-ever telephone call.<ref name="Watson1926"/> The genus name honors Watson and the species name honors Professor William Otis Crosby. Grabau initially interpreted ''Watsonella'' as a heteropod gastropod similar to the modern genus ''Carinaria''.<ref name="Grabau1900"/> In 1935, E. S. Cobbold named the genus ''Heraultia'' based on specimens from France, with the type species ''Heraultia varensalensis''. He tentatively interpreted it as a notostracan crustacean. He noted "striking" similarity between it and ''Watsonella'', but concluded that it was "impossible to bring them together" without more information on the affinities of ''Watsonella''.<ref name="Cobbold1935"/> Because the name ''Heraultia'' had already been used for a genus of fly, in 1976 John Pojeta and Bruce Runnegar proposed the name ''Heraultipegma'' as a replacement. Pojeta and Runnegar reinterpreted ''Heraultipegma'' and ''Watsonella'' as the earliest known rostroconchs.<ref name="Pojeta1976"/> In 1988, Martin Kerber hesitantly synonymized ''Heraultipegma'' and ''Watsonella'', and Ed Landing affirmed their synonymy in 1989.<ref name="Landing1989"/> In 2001, P. Yu. Parkhaev included ''Watsonella'' in Stenothecidae and proposed the subfamily Watsonellinae to include it.<ref name="Parkhaev2001"/>

Only one species of ''Watsonella'' is recognized as valid, the type species ''W. crosbyi''. The species ''Heraultia varensalensis'', ''Heraultia sibirica'', and ''Heraultipegma yunnanensis'' are all considered synonyms of ''W. crosbyi''.<ref name="Li2011"/> Another species, ''Heraultipegma charaulachica'', has been reassigned to the genus ''Xianfengella''.<ref name="Parkhaev2001"/>

==Description==

''Watsonella'' was a small, laterally-compressed animal, generally less than {{convert|5|mm|in}} long.<ref name="Landing1989"/> It had a single "pseudobivalved" shell.<ref name="Landing1989"/> It is not known with certainty which end of the shell is the front.<ref name="Guo2021"/>

The external surface of the shell was ornamented with comarginal growth lines and ribs.<ref name="Guo2021"/>

Unlike rostroconchs, ''Watsonella'' lacked a pegma, although the curvature of the shell may have served a functionally similar role.<ref name="Peel2021"/> Although Pojeta and Runnegar<ref name="Pojeta1976"/> and Landing<ref name="Landing1989"/> interpreted a pegma as present, several studies have rejected this interpretation.<ref name="Peel2021"/> Li and colleagues interpreted the putative pegma as an artifact of the curvature of the shell.<ref name="Li2011"/>

The shell of ''Watsonella'' was probably originally aragonitic in composition.<ref name="Landing1989"/> It was composed of two layers with distinct microstructure. The outer layer was composed of regular, tightly-packed prismatic units that were oriented perpendicular to the shell wall. The inner layer was composed of flattened units called lamellae that produced a stepwise texture.<ref name="Kouchinsky1999"/> Unlike many modern mollusks, the shell was not nacreous.<ref name="Vendrasco2011"/>

==Paleobiology== ''Watsonella'' was probably a burrowing animal. Because of the limited flexibility of its hingeless shell, its burrowing ability would have been less efficient than modern burrowing clams.<ref name="Landing1989"/>

==Evolutionary relationships== ''Anabarella'' was probably the ancestor of ''Watsonella''.<ref name="Vendrasco2011"/> Morphological intermediates linking ''Anabarella'' and ''Watsonella'' are known.<ref name="Gubanov1999"/> The shell microstructure is very similar between ''Anabarella'', ''Watsonella'', and the early bivalves ''Fordilla'' and ''Pojetaia''.<ref name="Vendrasco2011"/>

A phylogenetic analysis conducted in 2000 by a team of researchers led by J. G. Carter recovered ''Watsonella'' as more closely related to bivalves than rostroconchs.<ref name="Carter2000"/>

In 2023, a team of researchers led by Hao Song argued that ''Watsonella'' was too early to be a stem-group bivalve, based on molecular clock results that suggested that bivalves and scaphopods diverged from each other approximately 520 million years ago. They suggested that ''Watsonella'' should be reinterpreted as stem-group members of Diasoma, the clade uniting bivalves and scaphopods.<ref name="Song2023"/>

== Biostratigraphic significance== Watsonella has been proposed as an index fossil of the Cambrian, defining a W. crosbyi zone. Notwithstanding the weakness of a first appearance datum as a definition for the base of a period,<ref>1. Landing, E., Geyer, G., Brasier, M.D., and Bowring, S.A. (2013). Cambrian evolutionary radiation: context, correlation, and chronostratigraphy—overcoming deficiencies of the first appearance datum (FAD) concept. Earth-Science Rev. 123, 133–172.</ref> the species has been proposed as a marker for the base of the presently unratified second stage of the Terreneuvian (i.e. Cambrian Stage 2). However, the species has now been found late in the Fortunian, drawing back its first occurrence.<ref>Landing, E., and Kouchinsky, A. V. (2016). Correlation of the Cambrian Evolutionary Radiation: geochronology, evolutionary stasis of earliest Cambrian (Terreneuvian) small shelly fossil (SSF) taxa, and chronostratigraphic significance. Geol. Mag. 153, 750–756.</ref> But that said its occurrence in Australia seems to begin rather near the base of Stage 2.<ref name="Jacquet2017"/> ==References== {{Reflist|refs= <ref name="Carter2000">{{Cite journal| doi = 10.1144/GSL.SP.2000.177.01.04| issn = 0305-8719 | eissn = 2041-4927| volume = 177| issue = 1| pages = 47–79| last1 = Carter| first1 = J. G.| last2 = Campbell| first2 = D. C.| last3 = Campbell| first3 = M. R.| title = Cladistic perspectives on early bivalve evolution| journal = Geological Society, London, Special Publications| date = 2000 | bibcode = 2000GSLSP.177...47C }}</ref> <ref name="Cobbold1935">{{Cite journal| doi = 10.1080/00222933508655023| issn = 0374-5481| volume = 16| issue = 91| pages = 25–48| last = Cobbold| first = E.S.| title = Lower Cambrian faunas from Hérault, France| journal = Annals and Magazine of Natural History| date = 1935 }}</ref> <ref name="Grabau1900">{{Cite journal| volume = 4| issue = 3| last = Grabau| first = Amadeus William| author-link = Amadeus William Grabau | title = Palaeontology of the Cambrian Terranes of the Boston Basin| journal = Occasional Papers of the Boston Society of Natural History| date = 1900 | url = https://books.google.com/books?id=QGBIAAAAYAAJ }}</ref> <ref name="Gubanov1999">{{Cite journal| doi = 10.1111/j.1502-3931.1999.tb00534.x| issn = 0024-1164 | eissn = 1502-3931| volume = 32| issue = 2| pages = 155–157| last1 = Gubanov| first1 = Alexander P.| last2 = Kouchinsky| first2 = Artem V.| last3 = Peel| first3 = John S.| title = The first evolutionary-adaptive lineage within fossil molluscs| journal = Lethaia| date = 1999 | bibcode = 1999Letha..32..155G | url = https://www.idunn.no/doi/10.1111/j.1502-3931.1999.tb00534.x| url-access = subscription}}</ref> <ref name="Guo2021">{{Cite journal| doi = 10.1016/j.palwor.2020.04.006| issn = 1871-174X| volume = 30| issue = 1| pages = 1–19| last1 = Guo| first1 = Jun-Feng| last2 = Li| first2 = Guo-Xiang| last3 = Qiang| first3 = Ya-Qin| last4 = Song| first4 = Zu-Chen| last5 = Zhang| first5 = Zhi-Fei| last6 = Han| first6 = Jian| last7 = Wang| first7 = Wen-Zhe| title = Watsonella crosbyi from the lower Cambrian (Terreneuvian, Stage 2) Yanjiahe Formation in Three Gorges Area, South China| journal = Palaeoworld| date = 2021 }}</ref> <ref name="Jacquet2017">{{Cite journal| doi = 10.1017/S0016756816000704| issn = 0016-7568 | eissn = 1469-5081| volume = 154| issue = 5| pages = 1088–1104| last1 = Jacquet| first1 = Sarah M.| last2 = Brougham| first2 = Thomas| last3 = Skovsted| first3 = Christian B.| last4 = Jago| first4 = James B.| last5 = Laurie| first5 = John R.| last6 = Betts| first6 = Marissa J.| last7 = Topper| first7 = Timothy P.| last8 = Brock| first8 = Glenn A.| title = Watsonella crosbyi from the lower Cambrian (Terreneuvian, Stage 2) Normanville Group in South Australia| journal = Geological Magazine| date = 2017 | doi-access = free| bibcode = 2017GeoM..154.1088J }}</ref> <ref name="Kouchinsky1999">{{Cite journal| doi = 10.1111/j.1502-3931.1999.tb00537.x| issn = 0024-1164 | eissn= 1502-3931| volume = 32| issue = 2| pages = 173–180| last = Kouchinsky| first = Artem V.| title = Shell microstructures of the Early Cambrian Anabarella and Watsonella as new evidence on the origin of the Rostroconchia| journal = Lethaia| date = 1999 | bibcode = 1999Letha..32..173K }}</ref> <ref name="Landing1989">{{Cite journal| doi = 10.1017/S0022336000041196| issn = 0022-3360 | eissn = 1937-2337| volume = 63| issue = 5| pages = 566–573| last = Landing| first = Ed| title = Paleoecology and distribution of the Early Cambrian rostroconch Watsonella crosbyi Grabau| journal = Journal of Paleontology| date = 1989 | bibcode = 1989JPal...63..566L }}</ref> <ref name="Li2011">{{Cite journal| doi = 10.1111/j.1755-6724.2011.00400.x| issn = 1000-9515 | eissn = 1755-6724| volume = 85| issue = 2| pages = 309–319| last1 = Li| first1 = Guoxiang| last2 = Zhao| first2 = Xin| last3 = Gubanov| first3 = Alexander| last4 = Zhu| first4 = Maoyan| last5 = Na| first5 = Lin| title = Early Cambrian mollusc Watsonella crosbyi: A potential GSSP Index Fossil for the base of the Cambrian Stage 2| journal = Acta Geologica Sinica - English Edition| date = 2011-04-24| bibcode = 2011AcGlS..85..309L }}</ref> <ref name="Parkhaev2001">{{Cite book| isbn = 5-7846-0095-8| pages = 133–210| last = Parkhaev| first = P. 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Category:Cambrian fossil record Category:Extinct molluscs Category:Paleozoic life of Newfoundland and Labrador Category:Paleozoic life of Nova Scotia Category:Rostroconchia