Science - USA (2020-07-10)

facial geometry resembles that of jawless stem
gnathostomes,especially galeaspids ( 15 ). Re-
cent phylogenetic analyses have recovered anti-
archs,Brindabellaspis, and acanthothoracids
as basal to other jawed vertebrates ( 11 , 14 , 16 ),
a position they retain when the (not previ-
ously analyzed) genera discussed herein are
included in the analysis (fig. S1). The jaws
and dentition ofBrindabellaspisare unknown,
whereas antiarchs lack teeth ( 17 ). Thus, the
teeth of acanthothoracids have the unique
potential to illuminate the origin of vertebrate
dentition.
The tooth addition in the dentition of CPW.9,
which comprises a pair of supragnathal plates
on a snout (fig. S2) ( 12 ), has been described
as concentric ( 7 , 18 ). As a result, a detached
dermal tessera with concentrically arranged
odontodes from the same formation was mis-
interpreted as an acanthothoracid supragnathal
( 18 – 20 ). Reanalyzing the micro–computed
tomography (micro-CT) scans of CPW.9 reveals
that the supragnathals suture labially with the
dermal premedian plate. In fact, tooth addition
is radial from a labial founder region, and there
is no labial tooth addition (fig. S3). Instead, the
oldest small teeth in the founder region be-
come overgrown by larger dermal odontodes
as has been observed in the stem osteichthyan
Andreolepis( 2 ).
Using propagation phase contrast syn-
chrotron microtomography, we have discov-
ered the previously unknown dentitions of
Radotina,Kosoraspis, andTlamaspis( 21 ).
All differ substantially from that of CPW.9.
The dentition ofRadotina(Fig. 2) comprises
four rows of teeth carried on the ventral face
of a large dermal cheekbone sutured to the
external surface of the palatoquadrate. This
ventral face forms a longitudinal trough, with
the teeth carried on the labial flank and the
floor of the trough marked by a row of vascular
grooves. Thus, the dentition is unambiguously
marginal, located inside the jaw margin but
external to the palatoquadrate (Fig. 2C), and
separated from the external surface of the
cheek by a distinct labial edge of exposed bone
(fig. S4). Tooth rows are added lingually (fig.
S5), and a pronounced mineralization gradi-
ent is displayed from the oldest to youngest
teeth (fig. S4). The teeth have an elongated
bladelike stellate shape (Fig. 2E) that matches
that of marginal dermal ornament odontodes
on the tesserae of the cheek (fig. S6E). How-
ever, unlike those odontodes, the teeth are
fused into rows by a distinct basal attachment
tissue (fig. S7D). The arms of the stellate teeth,
which consist of pallial mesodentine, attach to
a central cone of circumpulpal semidentine
( 22 , 23 ). The large spaces of odontoblast clusters
enclosed in the arms anastomose with the
smaller and sparser cell spaces in the cone,
across the boundary between pallial and cir-
cumpulpal dentine (fig. S7, A to C). Each arm is

equipped with an enameloid-coated blade (fig.

S7D). Well-developed wear facets on the teeth

(fig. S5) show that this was a functional cutting

dentition. There is no evidence of resorption or

replacement of teeth, but a broken tooth in the

lingualmost row has been repaired with a new

crown inserted into the break (fig. S5, E and F).

Kosoraspis( 24 ) andTlamaspis( 21 ) have

multiple short tooth–bearing dermal jawbones.

They vary in length and shape according to

their position (fig. S8) but invariably bear a

prominent facial lamina. InKosoraspis, the

facial lamina carries toothlike odontodes that

grade into recurved pointed teeth on the oral

lamina (Fig. 3 and figs. S9 and S10), forming

obliquely transverse whorl-like files that re-

semble the first-generation toothlike odontodes

and nonshedding teeth inAndreolepis( 2 ). They

212 10 JULY 2020•VOL 369 ISSUE 6500 sciencemag.org SCIENCE

Osteostraci

Galeaspida

Brindabellaspis

Romundina

Buchanosteus

Entelognathus

Radotina

Kosoraspis

Tlamaspis

Lophosteus

Moythomasia

Squalus

Ptomacanthus

CPW.9

Kujdanowiaspis

Gemuendina

Janusiscus

palatoquadrate

external dermal bone

dermal odontode

bone of outer dental arcade

bone of inner dental arcade

entopterygoid

tooth

youngest tooth position

site-specific resorption

shedding

Macropetalichthys

Antiarchi

CHONDRICHTYES

OSTEICHTYES

STEM

GROUP

CROWN

GROUP

JAWED

JAWLESS

ARTHRODIRA

́ACANTHOTHORACI ́

Fig. 1. Distribution of dentition types among gnathostomes.Theimages represent schematic transverse

sections of palatoquadrate complexes, except for that of CPW.9, which represents the prenasal region.

Genera with these schematic images are noted in bold. For strict consensus phylogeny, see fig. S1. The

arthrodire and stem osteichthyan reconstructions are based on ( 2 , 9 , 27 ). The division between stem

and crown is based on ( 2 , 9 , 27 , 28 ), which resolveLophosteusas a stem osteichthyan andEntelognathus

andJanusiscusas stem gnathostomes.

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