Editing Tusk shell (section)

== Evolution ==

=== Fossil record ===
[[File:05898 Grand Canyon Mollusca Fossils (4739033749).jpg|thumb|[[Internal mold]] of a fossil scaphopod, [[Kaibab Formation]] ([[Grand Canyon]])]]
There is a good fossil record of scaphopods from the [[Mississippian (geology)|Mississippian]] onwards,<ref name="Yochelson20072">{{cite journal|author=Ellis L. Yochelson|author2=Royal H. Mapes|author3=Doris Heidelberger|year=2007|title=An enigmatic molluscan fossil from the Devonian of Germany: scaphopod or cephalopod?|journal=Paläontologische Zeitschrift|volume=81|issue=2|pages=118–122|doi=10.1007/BF02988386|bibcode=2007PalZ...81..118Y |s2cid=85124687}}</ref> making them the youngest molluscan class.

The Ordovician ''[[Rhytiodentalium]] kentuckyensis'' has been interpreted as an early antecedent of the scaphopods, implying an evolutionary succession from [[ribeirioid]] [[rostroconch]] molluscs such as ''[[Pinnocaris]]''.  However, a competing hypothesis suggests a Devonian/Carboniferous origin from a non-mineralized ancestor, or from a more derived, Devonian, [[conocardioid]] rostroconch.<ref name="Peel20042">{{Cite journal|author=Peel, J.S.|year=2004|title=''Pinnocaris'' and the origin of scaphopods.|journal=Acta Palaeontologica Polonica|volume=49|issue=4|pages=543–550}}</ref>

=== Phylogeny ===
The scaphopods are largely agreed to be members of the [[Conchifera]], however their phylogenetic relationship with the other members of this subphylum remains contentious. The Diasoma concept proposes a clade of scaphopods and bivalves based on their shared infaunal lifestyle, burrowing foot, and possession of a mantle and shell. Pojeta and Runnegar proposed the extinct [[Rostroconchia]] as the stem group of the Diasoma.<ref>{{cite book |doi=10.3133/pp968 |chapter=The paleontology of rostroconch mollusks and the early history of the phylum Mollusca |series=Professional Paper |year=1976 |last1=Pojeta |first1=John |last2=Runnegar |first2=Bruce }}</ref> An alternative hypothesis proposes the cephalopods and gastropods as sister to the scaphopods with [[helcionellid]]s as the stem group.<ref>{{Cite journal|last1=Stöger|first1=I.|last2=Sigwart|first2=J. D.|last3=Kano|first3=Y.|last4=Knebelsberger|first4=T.|last5=Marshall|first5=B. A.|last6=Schwabe|first6=E.|last7=Schrödl|first7=M.|date=2013|title=The Continuing Debate on Deep Molluscan Phylogeny: Evidence for Serialia (Mollusca, Monoplacophora + Polyplacophora)|journal=BioMed Research International|language=en|volume=2013|pages=407072|doi=10.1155/2013/407072 |pmc=3856133|pmid=24350268|doi-access=free}}</ref> A review of deep molluscan phylogeny in 2014 found more support for the scaphopods, gastropods, or cephalopods than for scaphopods and bivalves, thus the shared body features of scaphopods and bivalves may be convergent adaptations due to similar lifestyles.<ref>{{Cite journal|last1=Schrödl|first1=Michael|last2=Stöger|first2=Isabella|date=2014-12-25|title=A review on deep molluscan phylogeny: old markers, integrative approaches, persistent problems|journal=Journal of Natural History|volume=48|issue=45–48|pages=2773–2804|doi=10.1080/00222933.2014.963184|bibcode=2014JNatH..48.2773S |s2cid=84697563|issn=0022-2933}}</ref> Analysis of the scaphopod nervous system demonstrated that both scaphopods and cephalopods share a similar nervous system structure, with ventrally shifted pedal nerves and lateral nerves that extend dorsally. These similarities led to the conclusion that scaphopods are sister to the cephalopods with gastropods as sister to them both.<ref name=":0" /> More recent research, including the sequenced genome of tusk shells, support the Diasoma model with bivalves as the sister group.<ref>[https://www.eurekalert.org/news-releases/1001805 Genomes of enigmatic tusk shells provide new insights into early Molluscan evolution]</ref>