''Watsonella'' is an extinct genus of

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

known from early (Terreneuvian) Cambrian strata. It has been hypothesized to be close to the origin of

bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

. It contains a single species, ''Watsonella crosbyi''. The genus is closely related to ''

Anabarella ''Anabarella'' is a species of bilaterally-flattened monoplacophoran mollusc, with a morphological similarity to the rostroconchs. Its shell preserves evidence of three mineralogical textures on its outer surface: it is polygonal near the crest ...

'', with which it bears many morphological similarities, including a laminar internal shell microstructure said to connect it with the early

'' Fordilla'' and ''

Pojetaia ''Pojetaia'' is an extinct genus of early bivalves, one of two genera in the extinct family Fordillidae. The genus is known solely from Early to Middle Cambrian fossils found in North America, Greenland, Europe, North Africa, Asia, and Australi ...

''.

Taxonomy

''Watsonella'' was described by

Amadeus William Grabau Amadeus William Grabau (January 9, 1870 – March 20, 1946) was an American geologist, teacher, stratigrapher, paleontologist, and author who worked in the United States and China. Biography Grabau's grandfather, J.A.A. Grabau, led a group of ...

in 1900, with the type species ''Watsonella crosbyi''. 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. The genus name honors Watson and the species name honors Professor William Otis Crosby. Grabau initially interpreted ''Watsonella'' as a

heteropod The Pterotracheoidea is, according to the Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005), a taxonomic superfamily of sea snails or sea slugs, marine gastropod molluscs in the clade Littorinimorpha. They are commonly called heteropods or ...

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

similar to the modern genus '' Carinaria''. 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 Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

. 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''. Because the name '' Heraultia'' had already been used for a genus of

fly Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...

, 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 The Rostroconchia is a class of extinct molluscs dating from the early Cambrian to the Late Permian. They were initially thought to be bivalves, but were later given their own class. They have a single shell in their larval stage, and the adult ...

. In 1988, Martin Kerber hesitantly synonymized ''Heraultipegma'' and ''Watsonella'', and Ed Landing affirmed their synonymy in 1989. In 2001, P. Yu. Parkhaev included ''Watsonella'' in

Stenothecidae Stenothecidae is an extinct family of fossil univalved Cambrian molluscs which may be either gastropods or monoplacophorans. The name of this taxon should not be confused with that of the class Stenothecoida, a group of problematic Cambrian i ...

and proposed the subfamily Watsonellinae to include it. 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''. Another species, ''Heraultipegma charaulachica'', has been reassigned to the genus '' Xianfengella''.

Description

''Watsonella'' was a small, laterally-compressed animal, generally less than long. It had a single "pseudobivalved" shell. It is not known with certainty which end of the shell is the front. The external surface of the shell was ornamented with comarginal growth lines and ribs. Unlike rostroconchs, ''Watsonella'' lacked a pegma, although the curvature of the shell may have served a functionally similar role. Although Pojeta and Runnegar and Landing interpreted a pegma as present, several studies have rejected this interpretation. Li and colleagues interpreted the putative pegma as an artifact of the curvature of the shell. The shell of ''Watsonella'' was probably originally

aragonitic Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from ...

in composition. 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. Unlike many modern mollusks, the shell was not

nacreous Nacre ( , ), also known as mother-of-pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer. It is also the material of which pearls are composed. It is strong, resilient, and iridescent. Nacre is ...

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.

Evolutionary relationships

''Anabarella'' was probably the ancestor of ''Watsonella''. Morphological intermediates linking ''Anabarella'' and ''Watsonella'' are known. The shell microstructure is very similar between ''Anabarella'', ''Watsonella'', and the early bivalves '' Fordilla'' and ''

''. 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. 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 The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleot ...

results that suggested that bivalves and

scaphopods Scaphopoda (plural scaphopods , from Ancient Greek σκᾰ́φης ''skáphē'' "boat" and πούς ''poús'' "foot"), whose members are also known as tusk shells or tooth shells, are a class of shelled marine invertebrates belonging to the ph ...

diverged from each other approximately 520 million years ago. They suggested that ''Watsonella'' should be reinterpreted as stem-group members of

Diasoma Diasoma is a proposed clade of mollusks uniting the classes Scaphopoda and Bivalvia. Whether scaphopods and bivalves are each other's closest living relatives among mollusks is disputed, leaving the monophyly of Diasoma in doubt. Diasoma was origin ...

, the clade uniting bivalves and scaphopods.

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, 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.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. But that said its occurrence in Australia seems to begin rather near the base of Stage 2.

References

{{Taxonbar, from=Q28431115 Cambrian fossil record Extinct molluscs Paleozoic life of Newfoundland and Labrador Paleozoic life of Nova Scotia