Xiphograptus and the evolution of virgella‐bearing graptoloids

Abstract

The virgellar spine is one of the most consistent features of the graptolite sicula. It is present in a large number of graptoloid groups, but evolved separately and independently in these as it is seen from the presence of the spine in either ventral (Axonophora) or dorsal (Phyllograptus, Xiphograptus) position. The evolution of the virgellar spine in the Pan-Bireclinata in the Upper Dapingian to Lower Darriwilian time interval is known to follow four main steps, from a simple rutellum, through a lamelliform rutellum and a lanceolate virgella to the true virgellar spine. For the xiphograptids and in Phyllograptus, the origin and early development is less well documented, but appears to follow a similar path. However, the individual stages are condensed, and a true virgellar spine emerges already in the Floian time interval. A virgellar spine was found in Didymograptellus bifidus, necessitating a revision of the diagnosis of the genus Didymograptellus. A number of species of the virgellate genera Xiphograptus, Yutagraptus and Didymograptellus are described from isolated material for the first time. The species are useful for the biostratigraphic correlation of endemic mid-continent North American faunas with the Pacific Type faunal realm. Xiphograptus artus sp. nov., Didymograptellus primus sp. nov. and Didymograptellus cowheadensis sp. nov. from the Cow Head Group of western Newfoundland are described as new.

Ordovician graptolites from the basal part of the Palaeozoic transgressive sequence in the Karadere area, Zonguldak Terrane, NW Turkey; pp. 227–232

“…The figured specimens of 'Didymograptus (Didymograptellus) protobifidus' (Dean et al 2000, fig. 11f-h) have a short, wide sicula that may belong to Xiphograptus vdeflexus (Harris), a species recorded from the middle Dapingian (Dp2) Arienigraptus gracilis Biozone to the lower Darriwilian (Dw1) Levisograptus austrodentatus Biozone (Maletz 2010). This would mean a significantly younger (ca middle Dapingian) age for the base of the Karadere Formation, which was correlated by Dean et al (2000) Dean et al (2000) below the first appearance of L. austrodentatus in the Karadere Formation, should also be investigated.…”

Chapter 26 Graptolite palaeobiogeography

“…The axonophorans originated in deep, offshore environments from isograptid and pseudisograptid ancestors (Maletz 2010) and subsequently migrated into shallow-water regions, as can be shown from their distribution in the Upper Yangtze Region . The axonophorans originated in deep, offshore environments from isograptid and pseudisograptid ancestors (Maletz 2010) and subsequently migrated into shallow-water regions, as can be shown from their distribution in the Upper Yangtze Region .…”

“…The earliest xiphograptids appear with the genus Didymograptellus in the Tetragraptus akzharensis Biozone of western Newfoundland (Maletz 2010) as low-palaeolatidude endemics. The earliest xiphograptids appear with the genus Didymograptellus in the Tetragraptus akzharensis Biozone of western Newfoundland (Maletz 2010) as low-palaeolatidude endemics.…”

Climacograptus pungensRuedemann, 1904 and the definition of the Darriwilian (Ordovician) graptolite genusArchiclimacograptusMitchell, 1987

“…Thus, these features are useless for any detailed phylogenetic analysis unless a good understanding of their developmental origin and modification provides the evidence for their underlying homologies. The recognition of truly evolutionary homologous features is extremely difficult as is shown by the example of the polyphyletic origin of the virgellar spine (Maletz 2010), a simple rhabdosome feature with a complex constructional past; again a constructionally homologous, but phylogenetically analogous feature. For a cladistic analysis, this means that a character like the virgellar spine would have to be coded differently in the three established virgellate graptolite groups of independent origin to truly represent homologous characters.…”

“…The ventral virgellar spine, once gained in an early axonophoran ancestor (Maletz 2010) and retained through the whole later evolutionary history of the graptolites until their extinction in the Lower Devonian, is an excellent example of evolutionary stability of structural characters. The presence of apertural spines on the first thecal pair only, the prothecal folds, paired antivirgellar spines on the sicula, the position of the dicalycal theca and others, again, are highly conservative features and have been used for differentiating the main groups of the axonophorans in the past.…”