240
Min age. 514 Ma. The fossils derive from the Hongjingshao Member of the
Canglangpu Formation which is Cambrian Stage 3 in age, and younger than the
Chengjiang Biota (Zeng et al. 2014; Chen et al. 2020).
Annelid fossils of a similar age to those found in Chengjiang have been found in
the early Cambrian Sirius Passet Lagerstätte (Conway Morris and Peel 2008). Two
species are currently described from this deposit, namely Phragmochaeta canicularis (Conway Morris and Peel 2008) and Pygocirrus butyricampum (Vinther et al.
2011). Although annelid fossils are more abundant at this site than Chengjiang the
age of Sirius Passet is less tightly constrained. This deposit is constrained to the
middle to late Cambrian Stage 3 based on its correlation with the Nevadella trilobite
Zone (Harper et al. 2019). According to GTS 2016 (Ogg et al. 2016), this would
yield a minimum age of 515 Ma but the site may be as old as 518 Ma and so this
uncertainty encompasses the age of the Chengjiang Lagerstätten (Yang et al. 2018).
Previously, the annelid crown node has been calibrated based on the age of the
oldest scolecodonts (Benton et al. 2015), the jaws of eunicidan polychaetes, which
first appear unambiguously in the Tremadocian. Eunicida is deeply nested within
annelids however (Weigert et al. 2014; Parry et al. 2019a) and so this calibration
was always a conservative estimate for the age of the annelid crown group. The
above age for the annelid crown node may also be slightly too conservative if the
identification of sipunculans from the Chengjiang Biota is correct (Huang et al.
2004) as Sipuncula is recovered as an ingroup annelid taxon in phylogenomic analyses (Weigert et al. 2014). However, no additional sipunculan specimens from the
Cambrian have been positively identified (nor are unambiguous sipunculans known
from younger deposits) and so the occurrence of this taxon in the Cambrian requires
confirmation. Pygocirrus butyricampum from Sirius Passet could also conceivably
be the oldest crown group annelid, but the phylogenetic position of this taxon is
poorly constrained and it is typically resolved in a polytomy with the annelid crown
node (Parry et al. 2019a).
All of the fossil annelids known from the Cambrian either have morphological
features indicating an epibenthic lifestyle or live in dwelling tubes and so are
demonstrably free living and not parasitic. So far, fossil evidence for only four lineages of parasitic annelids has been identified: Branchiobdellida, Myzostomida,
Oenonidae and Spionidae. Other groups of annelid parasites have no known fossil record.
Two eunicidan polychaete families that share a prionognath type jaw apparatus
are parasitic (Paxton 2009). The family Oenonidae contains species that spend part
of their lifecycle as endoparasites of polychaetes, echiurans and bivalves (HernándezAlcántara and Solís-Weiss 1998). The majority of oenonids parasitize other polychaetes, although some species are thought to be exclusively free-living. This may
be a consequence of the parasitic phase of these species not having been recognized
however, and all oenonids may in fact share a parasitic lifestyle (Poulin 2001). The
Histriobdellidae share a similar jaw architecture and live as commensals on crustaceans, but their close relationship to Oenonidae and other prionognath eunicidans is
not universally accepted (Paxton 2009); some authors consider them as close
K. De Baets et al.
Min age. 514 Ma. The fossils derive from the Hongjingshao Member of the
Canglangpu Formation which is Cambrian Stage 3 in age, and younger than the
Chengjiang Biota (Zeng et al. 2014; Chen et al. 2020).
Annelid fossils of a similar age to those found in Chengjiang have been found in
the early Cambrian Sirius Passet Lagerstätte (Conway Morris and Peel 2008). Two
species are currently described from this deposit, namely Phragmochaeta canicularis (Conway Morris and Peel 2008) and Pygocirrus butyricampum (Vinther et al.
2011). Although annelid fossils are more abundant at this site than Chengjiang the
age of Sirius Passet is less tightly constrained. This deposit is constrained to the
middle to late Cambrian Stage 3 based on its correlation with the Nevadella trilobite
Zone (Harper et al. 2019). According to GTS 2016 (Ogg et al. 2016), this would
yield a minimum age of 515 Ma but the site may be as old as 518 Ma and so this
uncertainty encompasses the age of the Chengjiang Lagerstätten (Yang et al. 2018).
Previously, the annelid crown node has been calibrated based on the age of the
oldest scolecodonts (Benton et al. 2015), the jaws of eunicidan polychaetes, which
first appear unambiguously in the Tremadocian. Eunicida is deeply nested within
annelids however (Weigert et al. 2014; Parry et al. 2019a) and so this calibration
was always a conservative estimate for the age of the annelid crown group. The
above age for the annelid crown node may also be slightly too conservative if the
identification of sipunculans from the Chengjiang Biota is correct (Huang et al.
2004) as Sipuncula is recovered as an ingroup annelid taxon in phylogenomic analyses (Weigert et al. 2014). However, no additional sipunculan specimens from the
Cambrian have been positively identified (nor are unambiguous sipunculans known
from younger deposits) and so the occurrence of this taxon in the Cambrian requires
confirmation. Pygocirrus butyricampum from Sirius Passet could also conceivably
be the oldest crown group annelid, but the phylogenetic position of this taxon is
poorly constrained and it is typically resolved in a polytomy with the annelid crown
node (Parry et al. 2019a).
All of the fossil annelids known from the Cambrian either have morphological
features indicating an epibenthic lifestyle or live in dwelling tubes and so are
demonstrably free living and not parasitic. So far, fossil evidence for only four lineages of parasitic annelids has been identified: Branchiobdellida, Myzostomida,
Oenonidae and Spionidae. Other groups of annelid parasites have no known fossil record.
Two eunicidan polychaete families that share a prionognath type jaw apparatus
are parasitic (Paxton 2009). The family Oenonidae contains species that spend part
of their lifecycle as endoparasites of polychaetes, echiurans and bivalves (HernándezAlcántara and Solís-Weiss 1998). The majority of oenonids parasitize other polychaetes, although some species are thought to be exclusively free-living. This may
be a consequence of the parasitic phase of these species not having been recognized
however, and all oenonids may in fact share a parasitic lifestyle (Poulin 2001). The
Histriobdellidae share a similar jaw architecture and live as commensals on crustaceans, but their close relationship to Oenonidae and other prionognath eunicidans is
not universally accepted (Paxton 2009); some authors consider them as close
K. De Baets et al.