126
fossils show definitive chordate characters, including gill slits, fins, and metameric
musculature. The provenance of many other proposed vertebrate fossils is more
controversial. For example, Haikouella or Yunnanozoon could be fossil hemichordates, acranians (vertebrates lacking skulls), stem deuterostomes or stem vertebrates
(Donoghue and Keating 2014). Several attempts have been made to date the vertebrate phylogeny using molecular clocks. Most studies, however, focus on the gnathostomes for which more fossil calibration points exist, leaving the origin of the
vertebrates relatively uncertain. Dohrmann and Wörheide (2017) place the deuterostome origin before the Marinoan glaciation and chordate and vertebrate origins
into Cambrian and Ordovician, respectively. The origin of (crown) vertebrates has
been variously placed at 490 (504–476) Ma (Delsuc et al. 2018), 460–533 Ma (dos
Reis et al. 2015) and 480 Ma (Holzer et al. 2018).
Recent craniate vertebrates include the jawless lampreys and hagfishes, the
chondrichthyans (cartilaginous fishes), and the osteichthyans (bony fishes), the latter comprising sarcopterygians and actinopterygians. The Agnatha (or Cyclostomata)
(jawless fishes) are currently considered to be monophyletic, forming the sister
group to Gnathostomata (vertebrates with jaws, including most present-day fish).
Because of the complete lack of hard skeletons the origin of agnathans is difficult to
trace; the earliest reliable fossils are from the Devonian and Carboniferous.
Molecular clock studies converge on ~460 Ma for the gnathostome divergence
(Irisarri et al. 2017; dos Reis et al. 2015; Broughton et al. 2013).
Two further exclusively fossil groups are of relevance for early vertebrate relationships: the conodonts and the ostracoderms. Conodonts are known by their characteristic tooth-like skeletal elements from the late Cambrian until the Late Triassic
when they apparently went extinct. Currently conodonts are mostly considered to
branch at the base of vertebrates, either as stem agnathans or stem gnathostomes
(Goudemand et al. 2011; Murdock et al. 2013; Turner et al. 2010). The ostracoderms (Placodermi, Osteostraci, Acanthodii, with origins estimated from 467 to
295 Ma) are interpreted as a paraphyletic assemblage of gnathostome stem-lineage
taxa that mostly lacked jaws (Janvier 2001). The earliest lobe-fin fossils are from
423 Ma (Brazeau and Friedman 2015), marking the putative chondrichthyan/osteichthyan (crown-gnathostome) divergence. Chondrichthyan-like scales are known
from as early as the late Ordovician (458 Ma; Andreev et al. 2016). Vertebrates with
chondrichthyan body fossils, however, do not appear in the fossil record before
400 Ma (Brazeau and Friedman 2015). The first definitive actinopterygian,
Cheirolepis, is from the early Middle Devonian (~390 Ma), although isolated scales
of ray-fins were present from ~427 Ma (Friedman 2015). Molecular clock analyses
suggest crown Actinopterygii to have been present by 400 Ma (Broughton et al.
2013), 384 Ma (Near et al. 2012), or 320 Ma (Irisarri et al. 2017) and ages for teleosts to range from 300 to 200 Ma. Recent Actinopterygii include the three sequentially branching taxa Cladistia (bichirs and ropefish), Chondrostei (sturgeon and
paddlefish), and Holostei (gars and bowfin). The most derived actinopterygians, the
Teleostei, includes the majority of the ~30,000 actinopterygian species (Friedman
2015). Only scattered actinopterygian fossils are known from the Devonian indicating relatively cryptic evolution until several radiations took place after the Devonian/
B. Okamura and A. Gruhl
fossils show definitive chordate characters, including gill slits, fins, and metameric
musculature. The provenance of many other proposed vertebrate fossils is more
controversial. For example, Haikouella or Yunnanozoon could be fossil hemichordates, acranians (vertebrates lacking skulls), stem deuterostomes or stem vertebrates
(Donoghue and Keating 2014). Several attempts have been made to date the vertebrate phylogeny using molecular clocks. Most studies, however, focus on the gnathostomes for which more fossil calibration points exist, leaving the origin of the
vertebrates relatively uncertain. Dohrmann and Wörheide (2017) place the deuterostome origin before the Marinoan glaciation and chordate and vertebrate origins
into Cambrian and Ordovician, respectively. The origin of (crown) vertebrates has
been variously placed at 490 (504–476) Ma (Delsuc et al. 2018), 460–533 Ma (dos
Reis et al. 2015) and 480 Ma (Holzer et al. 2018).
Recent craniate vertebrates include the jawless lampreys and hagfishes, the
chondrichthyans (cartilaginous fishes), and the osteichthyans (bony fishes), the latter comprising sarcopterygians and actinopterygians. The Agnatha (or Cyclostomata)
(jawless fishes) are currently considered to be monophyletic, forming the sister
group to Gnathostomata (vertebrates with jaws, including most present-day fish).
Because of the complete lack of hard skeletons the origin of agnathans is difficult to
trace; the earliest reliable fossils are from the Devonian and Carboniferous.
Molecular clock studies converge on ~460 Ma for the gnathostome divergence
(Irisarri et al. 2017; dos Reis et al. 2015; Broughton et al. 2013).
Two further exclusively fossil groups are of relevance for early vertebrate relationships: the conodonts and the ostracoderms. Conodonts are known by their characteristic tooth-like skeletal elements from the late Cambrian until the Late Triassic
when they apparently went extinct. Currently conodonts are mostly considered to
branch at the base of vertebrates, either as stem agnathans or stem gnathostomes
(Goudemand et al. 2011; Murdock et al. 2013; Turner et al. 2010). The ostracoderms (Placodermi, Osteostraci, Acanthodii, with origins estimated from 467 to
295 Ma) are interpreted as a paraphyletic assemblage of gnathostome stem-lineage
taxa that mostly lacked jaws (Janvier 2001). The earliest lobe-fin fossils are from
423 Ma (Brazeau and Friedman 2015), marking the putative chondrichthyan/osteichthyan (crown-gnathostome) divergence. Chondrichthyan-like scales are known
from as early as the late Ordovician (458 Ma; Andreev et al. 2016). Vertebrates with
chondrichthyan body fossils, however, do not appear in the fossil record before
400 Ma (Brazeau and Friedman 2015). The first definitive actinopterygian,
Cheirolepis, is from the early Middle Devonian (~390 Ma), although isolated scales
of ray-fins were present from ~427 Ma (Friedman 2015). Molecular clock analyses
suggest crown Actinopterygii to have been present by 400 Ma (Broughton et al.
2013), 384 Ma (Near et al. 2012), or 320 Ma (Irisarri et al. 2017) and ages for teleosts to range from 300 to 200 Ma. Recent Actinopterygii include the three sequentially branching taxa Cladistia (bichirs and ropefish), Chondrostei (sturgeon and
paddlefish), and Holostei (gars and bowfin). The most derived actinopterygians, the
Teleostei, includes the majority of the ~30,000 actinopterygian species (Friedman
2015). Only scattered actinopterygian fossils are known from the Devonian indicating relatively cryptic evolution until several radiations took place after the Devonian/
B. Okamura and A. Gruhl