Downloaded from http://rspb.royalsocietypublishing.org/ on June 18, 2017
Proc. R. Soc. B (2012) 279, 775–779
doi:10.1098/rspb.2011.1107
Published online 20 July 2011
First tooth-set outside the jaws in a
vertebrate
John A. Finarelli1,* and Michael I. Coates1,2
1
Department of Organismal Biology and Anatomy, and 2Committee on Evolutionary Biology, University of
Chicago, Chicago, IL 60637, USA
Holocephalans (ratfish, rabbitfish and chimaeras) figure with increasing prominence in studies of gnathostome evolutionary biology. Here, we provide the first complete description of the teeth and toothplates
of one of the earliest known holocephalans, Chondrenchelys problematica, including the first unambiguous
evidence of a gnathostome with an extra-mandibular dentition. We further demonstrate that holocephalan
toothplate ontogeny differs fundamentally from all other extant gnathostome examples, and show how
the conjunction of these teeth and toothplates challenges the monophyly of an extinct chondrichthyan
clade, the Petalodontiformes. Chondrenchelys provides a novel perspective on the evolution of dentitions
in shark-like fishes, expands the known repertoire of gnathostome dental morphologies and offers a glimpse
of radically new chondrichthyan ecomorphs, now lost from the modern biota, following the end-Devonian
extinctions.
Keywords: Chondrichthyes; Holocephali; extra-mandibular teeth; Palaeozoic
1. INTRODUCTION
Living chimaeroids are the remnants of a major Palaeozoic
radiation of cartilaginous fishes. As such, they represent
a significant component of extant gnathostome diversity
and, in terms of their morphological and genomic heritage, provide unique insights into conditions among
early vertebrates [1,2]. Genome sequencing of the elephant shark (Callorhinchus milii) [3] has raised interest in
holocephalans as a model system in comparative evolutionary biology, evolutionary developmental biology
and genomics [4 –6]. Extant holocephalans are characterized by a distinctive suite of anatomical specializations
[7– 10]; however, we have little appreciation of the deep
evolutionary history of this clade and its diagnostic
features.
Most Palaeozoic holocephalans are known only from
isolated teeth [9], and the few known body fossils present
strikingly different ecomorphs compared with those
observed today [7,11,12]. In practice, the parallel, and
often independent, treatments of data drawn from the
abundant record of isolated teeth, on the one hand
[13], and the more rarely preserved articulated skeletal
remains, on the other [14,15], present a persistent problem in the study of early vertebrate diversity. Here,
we describe the dentition of the Lower Carboniferous
(Viséan) holocephalan Chondrenchelys problematica
(Traquair [16]; figure 1), using new and previously unrecognized specimens. We provide (i) the first description of
a complete arcade of extra-mandibular teeth, (ii) a new
description of holocephalan toothplate ontogeny, and
(iii) the conjunction, within a single species, of characteristic holocephalan toothplates with individual,
petalodont-like teeth. These novel features have important implications for the evolution of disparity in
modern gnathostome dentitions, interpretations of the
patterns of early fossil vertebrate diversity from records
of isolated teeth and the range of ecomorphs among
early chondrichthyans.
Chondrenchelys problematica, from Glencartholm,
Scotland [17], was first described in detail by MoyThomas [11]. Lund [12] amended its description while
describing a slightly younger (Serpukhovian) taxon,
Harpagofututor volsellorhinus from the Bear Gulch Limestone of Montana, USA. Lund [12] allied Chondrenchelys
and Harpagofututor in the family Chondrenchelyidae
based on general similarities of their eel-like body plans,
specialized pectoral fins and toothplate morphologies.
2. MATERIAL AND METHODS
The new Mumbie Quarry specimens, NMS 1998.35.1
and NMS 2002.68.1, are more complete than previously
described examples, and benefit from more precise preparation and consolidation. NMS 1998.35.1 is a single
slab containing an unusually intact individual with the
upper and lower dentitions preserved in occlusion. NMS
2002.68.1 is preserved as a part and counterpart. The specimen is split through the oral cavity, such that both the
mandibular and palatal dentitions are preserved in oral
view. The previously misidentified specimen, HM V.7173
(see the electronic supplementary material), is a single
slab preserving the dentition primarily in a dorsal view,
with the posterior toothplates preserved in occlusion and
damaged anterior toothplates, displaced rostrally. New data
preserved in these specimens motivated a re-evaluation of
the dentitions in previously described Chondrenchelys specimens (including NHM P 4085 and RSM 1885.54.5), part
of Moy-Thomas’s [11] original description, and referred
specimens RMS 1891.53.33 and BGS-GSE 13328 [12].
The wealth of new information from these specimens
facilitated the complete redescription of the dentition of
Chondrenchelys (see the electronic supplementary material
for complete description).
*Author for correspondence ([email protected]).
Electronic supplementary material is available at http://dx.doi.org/
10.1098/rspb.2011.1107 or via http://rspb.royalsocietypublishing.org.
Received 25 May 2011
Accepted 29 June 2011
775
This journal is q 2011 The Royal Society
Downloaded from http://rspb.royalsocietypublishing.org/ on June 18, 2017
776 J. A. Finarelli & M. I. Coates
(a)
Extra-mandibular holocephalan teeth
(b)
(c)
Figure 1. Comparison of mandibular dentitions for (a) chimaeroid, rabbitfish (Hydrolagus colliei); (b) Chondrenchelys problematica; and (c) general elasmobranch lyodont condition.
(b)
(a)
LPT
E-MT
UPT
UPT
UAT
LPT
LAT
(c)
(d)
key.r.
key.r.
Figure 2. (a,b) Anterior dentition of NMS 1998.35.1: (a) photo and (b) interpretive drawing. Anterior to the left in both. LPT,
lower posterior toothplate; LAT, lower anterior toothplate; UPT, upper posterior toothplate; UAT, upper anterior toothplate;
E-MT, extra-mandibular teeth. (c,d) Detail of extra-mandibular teeth in NMS 1998.35.1 showing dimpled surface and saddleshaped crowns. (c) Photo and (d) interpretive drawing. Anterior to the left and lingual to the bottom in both. key.r., keyholeshaped recess on lingual surface.
3. RESULTS
The new specimens demonstrate unambiguously the
presence of an extra-mandibular dentition, consisting of
individual teeth (figures 1b and 2; electronic supplementary material, figures S3 – 5), with complete mandibular
and palatal cartilages preserved, an unusual quality of preservation for Palaeozoic holocephalans. The outer surfaces
of these extra-mandibular teeth have a pin-cushion
appearance, reflecting vertically oriented, closely spaced
tubules emerging onto the outermost dentine surface
[9,13,18] (figure 2; electronic supplementary material,
figure S4). The crowns are labio-lingually compressed,
and the tooth bases are deep and likewise labio-lingually
thin (electronic supplementary material, figure S4). This
combination of characters closely resembles that of taxa
associated with the Petalodontiformes [9,13,18].
Proc. R. Soc. B (2012)
Each quadrant of both the mandibular and palatal
dentitions consists of a large posterior toothplate and a
single, smaller anterior toothplate (figure 3). Upper and
lower posterior toothplates (UPT and LPT, respectively;
electronic supplementary material) preserve complete
developmental sequences and display distinct shifts in
the orientation of tooth generation (figure 3). Transverse
dentine bands in the anteriormost portion of the LPT
and UPT, and the mineralized tritors within the
anterior-most ridge, show that the oldest part of each
toothplate was generated linguo-labially as in Helodus
and chochliodonts [8,9,19] (electronic supplementary
material, figure S2). In the posterior (younger) portion
of each toothplate (figure 3), the dentine bands are
reoriented, such that they pass transversely across the
toothplate.
Downloaded from http://rspb.royalsocietypublishing.org/ on June 18, 2017
Extra-mandibular holocephalan teeth
(a)
(b)
(c)
tri.r.
dnt.b
Figure 3. The large posterior toothplates of Chondrenchelys in
occlusal view. (a) Lower posterior toothplate (NMS
2002.68.1), (b) upper posterior toothplate (BGS-GSE
13328) and (c) map of the occlusion of the upper and
lower toothplate dentitions in Chondrenchelys. dnt.b., transverse dentine bands showing tooth generations; tri.r.,
tritural ridges demarcating tooth families.
4. DISCUSSION
Chondrenchelys possesses a set of individual teeth in conjunction with toothplates external to the mandibular arch
and distinct from the mandibular toothplate dentition
(see the electronic supplementary material). Information
from the new specimens and reinspection of previously
described material establish that an extra-mandibular dentition is a pervasive, and previously unrecognized, feature
of Chondrenchelys. Apparent extra-oral teeth are known
from several extant fish lineages [20,21], but these denticle-like structures cover portions of the dermal skeleton
and are not organized into functional arcades [21]. We
also note that extra-mandibular teeth have been reported
for several fossil taxa, but none of these is unambiguously external to the mandibular arch (see the electronic
supplementary material).
Functional tooth series are known from the pharynx
(branchial and/or hyoid arches) and the mandibular arch
(Meckel’s cartilage, the palatoquadrate and associated
dermal bones) in gnathostomes [22]. Chondrenchelys, like
other holocephalans, shows no trace of a pharyngeal dentition; yet there is evidence for a dentition external to the
mandibular arch, possibly supported on labial cartilages.
Gnathostome teeth are formed by odontogenic neural
crest deployed in a variety of positions throughout the
oral cavity [22,23]. Chondrenchelys now provides evidence
for the odontogenic potential of extra-mandibular neural
crest cell populations [24,25].
Tooth generation and replacement in Chondrichthyes
are often modelled as a generalized version of the lyodont
Proc. R. Soc. B (2012)
J. A. Finarelli & M. I. Coates
777
condition of elasmobranchs (figure 1c). Lyodont (‘loose
tooth’) dentitions are characterized by individual, unrooted
teeth, generated in a dental lamina that is set in a trough
along the lingual margin of the jaw cartilage [22]. Later
teeth sequentially replace earlier ones at each tooth
generation site, or family, with replacement proceeding
linguo-labially [19,22], more or less perpendicular to the
tangent of the tooth row. This pattern has been proposed
for Palaeozoic holocephalans, of which the scrolled toothplates of cochliodonts (electronic supplementary material,
figure S2) are the most striking examples [8,19].
In modern chimaeroids, tooth replacement is described as proceeding in a caudal-to-rostral direction [26].
Individual tooth units are thought to traverse the jaw
cartilage margins, with earlier generations abraded and
replaced anteriorly [19,26]. Consequently, it appears
that tooth replacement has been reoriented in holocephalans, from a primitively radial to a modern caudal–rostral
direction [9,19] (compare figure 1a,c). The Carboniferous
holocephalan Helodus simplex possesses distinct families of
mostly separate teeth, successively replacing one another
linguo-labially, as in extant sharks. But it also possesses
tooth families that are fused into incipient toothplates
(compare electronic supplementary material, figure S2a
with figure 1c). As such, Helodus has been presented as a
transitional form between the presumed ancestral and
derived conditions [8,18].
The new data on Chondrenchelys toothplate ontogeny
call this long-accepted scenario into question. The orientations of the transverse dentine bands and mineralized
tritors in the posterior toothplates show that the direction
of toothplate growth was reoriented during life.
In Chondrenchelys, the older (anterior) portion of the posterior toothplates is generated radially with transverse
bands oriented similarly to modern elasmobranch teeth,
whereas the younger (caudal) portion matches modern
chimaeroids, with transverse bands oriented in coronal
section, thus perpendicular to an antero-posterior direction of tooth generation (figure 3a; electronic
supplementary material, figure S3). Thus, within any posterior toothplate, the conjectured primitive and derived
conditions are observable. Radial replacement is probably
plesiomorphic for Chondrichthyes [1,22], but invoking a
straightforward transformation from unfused, radially
replacing teeth to caudal-to-rostrally replacing toothplates
is insufficient.
Gross toothplate morphology in Mesozoic and Cenozoic
holocephalans is essentially identical to Chondrenchelys (compare figure 3a with electronic supplementary material, figure
S2c). Within the LPT of Chondrenchelys, three reinforced
tritural ridges are arranged in what appear to be tooth
family whorls (figure 3a; electronic supplementary material,
figure S3). The anteriormost ridge is oriented labio-lingually,
and obliquely to the more posterior ridges, which fan out in
an increasingly parasagittal array. This is similar to more
recent holocephalans, such as the Mesozoic Myriacanthidae
and the extant families Callorhynchidae and Rhinochimaeridae [8,9]. However, these ridge patterns are unlike those
observed in the classic Palaeozoic holocephalan groups,
such as Chochliodontidae, Psammodontidae or Helodontidae [8,9], in which whorl orientations are strictly radial
(compare electronic supplementary material, figure S2a,b
with electronic supplementary material, figure S2c and
figure 3a).
Downloaded from http://rspb.royalsocietypublishing.org/ on June 18, 2017
778 J. A. Finarelli & M. I. Coates
Extra-mandibular holocephalan teeth
If elasmobranchs exemplify the primitive condition,
then the lyodont model would imply net movement of individual teeth relative to the jaw cartilage [19,26]. In
Chondrenchelys, the anteriormost portions of the posterior
toothplates remain in position throughout life (figure 3a,b).
This demonstrates that the posterior toothplates grew by
means of accretion of later tooth generations at the caudal
toothplate margin. There is no net movement of individual
teeth relative to the jaw cartilage. Further evidence for this
is provided in the mandibular cartilage flooring the LPT,
where a periodic structure coinciding with the transverse
dentine bands is observed (electronic supplementary
material, figure S3). We interpret this as coordinated,
episodic growth between the mandible and LPT. Intriguingly, toothplate growth via accretion resembles patterns
observed in several evolutionarily remote clades (e.g.
dipnoan sarcopterygians [27,28] and arthordire placoderms
[29]). Caudal accretion further emphasizes the fundamental
dissimilarity between the dentitions of Chondrenchelys and
conventional sharks [22,29].
The extra-mandibular dentition displays characters
that resemble teeth normally ascribed to the Petalodontiformes [9,13,18]. Petalodonts are an enigmatic group of
Palaeozoic chondrichthyans of uncertain phylogenetic affinity [13,15,18,30] (electronic supplementary material,
figure S1). In particular, we note that the extra-mandibular
teeth are strikingly similar to the teeth of Heteropetalus and
Debeerius (electronic supplementary material, figure S6),
both of which have been allied with or included within the
Petalodontiformes [13,31]. The similarities raise important
questions about the coherence of the Petalodontiformes
and the characters that define petalodont teeth, and more
generally about the diagnostic value of isolated teeth in
the early vertebrate fossil record. The conjunction of two
morphologies that were previously thought to be disjunct
serves as a cautionary example for erecting classifications and phylogenies based solely on dental character
data [13,32]. ‘Tooth-taxa’ and dental character phylogenies
are not new to palaeontology; teeth are the most easily
fossilized elements of many vertebrate skeletons, including
chondrichthyans. Several studies in mammalian palaeobiology have pointed to elevated rates of homoplasy and
incongruent phylogenetic results when considering dental
characters in isolation from other data partitions [33,34].
Given the observation of petalodont-like teeth in this otherwise decidedly holocephalan mouth, petalodonts probably
constitute a range of tooth forms, some of which are present
among the dentitions of a variety of early chondrichthyans.
As a result, it is possible that the Petalodontiformes is not
monophyletic, and we predict that an increasing number
of species will be removed from it as additional body fossils
with more or less ‘petalodont’ dentitions emerge in the
Palaeozoic chondrichthyan record.
Chondrenchelys raises new questions about the evolution of jawed vertebrate dentitions in the wake of the
end-Devonian extinction [35]. Furthermore, it reveals
experimentation with ecomorphologies that are never again
observed in the Holocephali. Dental and histological character data are important, but early vertebrate phylogeny
cannot be successfully resolved using these materials alone.
The importance of tooth, scale and histological data can
only be fully appreciated when understood in context with
more inclusive sets of character data, hence the importance
of fossils such as Chondrenchelys.
Proc. R. Soc. B (2012)
For access to specimens we thank: the British Geological
Survey of Edinburgh, J. Liston, N. Clark, C. Beard,
A. Henrici, R. Paton and S. Walsh. We acknowledge S. P.
Wood for discovery and primary preparation of the Mumbie
Quarry specimens and academic collaboration. We thank
L. C. Sallan for identification of the Hunterian Museum
specimen and B. Masek for painstaking preparation of the
new specimens. This project was supported by National
Science Foundation grant DEB-0917922.
REFERENCES
1 Brazeau, M. D. 2009 The braincase and jaws of a Devonian ‘acanthodian’ and modern gnathostome origins.
Nature 457, 305 –308. (doi:10.1038/nature07436)
2 Inoue, J. G., Miya, M., Lam, K., Tay, B. H., Danks, J. A.,
Bell, J., Walker, T. I. & Venkatesh, B. 2010 Evolutionary
origin and phylogeny of the modern holocephalans
(Chondrichthyes: Chimaeriformes): a mitogenomic perspective. Mol. Biol. Evol. 27, 2576–2586. (doi:10.1093/
molbev/msq147)
3 Venkatesh, B. et al. 2007 Survey sequencing and comparative analysis of the elephant shark (Callorhinchus milii)
genome. PLoS Biol. 5, 932– 944. (doi:10.1371/journal.
pbio.0050101)
4 Ravi, V., Lam, K., Tay, B. H., Tay, A., Brenner, S. &
Venkatesh, B. 2009 Elephant shark (Callorhinchus milii)
provides insights into the evolution of Hox gene clusters
in gnathostomes. Proc. Natl Acad. Sci. USA 106,
16 327–16 332. (doi:10.1073/pnas.0907914106)
5 Niimura, Y. 2009 On the origin and evolution of vertebrate olfactory receptor genes: comparative genome
analysis among 23 chordate species. Genome Biol. Evol.
1, 34–44. (doi:10.1093/gbe/evp003)
6 MacDonald, R. B., Debiais-Thibaud, M., Martin, K.,
Poitras, L., Tay, B. H., Venkatesh, B. & Ekker, M. 2010
Functional conservation of a forebrain enhancer from
the elephant shark (Callorhinchus milii ) in zebrafish and
mice. BMC Evol. Biol. 10, 157. (doi:10.1186/14712148-10-157)
7 Didier, D. 1995 Phylogenetic systematics of extant
chimaeroid fishes (Holocephali, Chimaeroidei). Am.
Mus. Nov. 3119, 1–86.
8 Patterson, C. 1965 Phylogeny of the chimaeroids. Phil.
Trans. R. Soc. Lond. B 249, 101 –219. (doi:10.1098/
rstb.1965.0010)
9 Stahl, B. J. 1999 Chondrichthyes III: Holocephali. In
Handbook of Paleoichthyology (ed. H.-P. Schultze), pp.
164. München, Germany: Verlag Dr. Friedrich Pfeil.
10 Gillis, J. A., Rawlinson, K. A., Bell, J., Lyon, W. S.,
Baker, C. V. H. & Shubin, N. H. 2011 Holocephalan
embryos provide evidence for gill arch appendage
reduction and opercular evolution in cartilaginous
fishes. Proc. Natl Acad. Sci. USA 108, 1507–1512.
(doi:10.1073/pnas.1012968108)
11 Moy-Thomas, J. A. 1935 The structure and affinities
of Chondrenchelys problematica Tr. Proc. Zool. Soc.
Lond. 105, 391 –404. (doi:10.1111/j.1469-7998.1935.
tb06256.x)
12 Lund, R. 1982 Harpagofututor volsellorhinus: new genus
and species (Chondrichthyes, Chondrenchelyiformes)
from the Namurian Bear Gulch Limestone, Chondrenchelys problematica Traquair (Viséan) and their
sexual dimorphism. J. Paleontol. 56, 938 –958.
13 Ginter, M., Hampe, O. & Duffin, C. 2010 Chondrichthyes: Paleozoic Elasmobranchii: teeth. Handbook of
Paleoichthyology (ed. H.-P. Schultze). München,
Germany: Verlag Dr. Friedrich Pfeil.
14 Coates, M. I. & Sequeira, S. E. K. 2001 A new stethacanthid
chondrichthyan from the Lower Carboniferous of Bearsden,
Downloaded from http://rspb.royalsocietypublishing.org/ on June 18, 2017
Extra-mandibular holocephalan teeth
15
16
17
18
19
20
21
22
23
24
25
Scotland. J. Vert. Paleontol. 21, 438–459. (doi:10.1671/
0272-4634(2001)021[0438:ANSCFT]2.0.CO;2)
Lund, R. & Grogan, E. D. 1997 Relationships of the
Chimaeriformes and the basal radiation of the Chondrichthyes. Rev. Fish Biol. Fisheries 7, 1–59. (doi:10.1023/
A:1018471324332)
Traquair, R. H. 1888 Further notes on Carboniferous
Selachii. Geol. Mag. 5, 101– 104. (doi:10.1017/S00167
56800173467)
Schram, F. 1983 Lower Carboniferous biota of
Glencartholm, Eskdale, Dumfriesshire. Scott. J. Geol.
19, 1–15. (doi:10.1144/sjg19010001)
Janvier, P. 1996 Early vertebrates.Oxford monographs on
geology and geophysics, No. 33. Oxford, UK: Clarendon
Press.
Patterson, C. 1992 Interpretation of the toothplates of
chimaeroid fishes. Zool. J. Linn. Soc. 106, 33–61.
(doi:10.1111/j.1096-3642.1992.tb01239.x)
Sire, J. Y. 2001 Teeth outside the mouth in teleost
fishes: how to benefit from a developmental accident.
Evol. Dev. 3, 104 –108. (doi:10.1046/j.1525-142x.2001.
003002104.x)
Sire, J. Y. & Allizard, F. 2001 A fourth teleost lineage possessing extra-oral teeth: the genus Atherion (Teleostei;
Atheriniformes). Eur. J. Morphol. 39, 295 –305. (doi:10.
1076/ejom.39.5.295.7375)
Smith, M. M. & Coates, M. I. 1998 Evolutionary
origins of the vertebrate dentition: phylogenetic patterns
and developmental evolution. Eur. J. Oral Sci. 106,
482 –500. (doi:10.1046/j.0909-8836.t01-4-.x)
Fraser, G. J., Cerny, R., Soukup, V., Bronner-Fraser, M.
& Streelman, J. T. 2010 The odontode explosion: the
origin of tooth-like structures in vertebrates. Bioessays
32, 808– 817. (doi:10.1002/bies.200900151)
Kimmel, C. B. & Eberhart, J. K. 2008 The midline, oral
ectoderm, and the arch-0 problem. Integr. Comp. Biol. 48,
668 –680. (doi:10.1093/icb/icn048)
Cerny, R., Cattell, M., Sauka-Spengler, T., BronnerFraser, M., Yu, F. & Medeiros, D. M. 2010 Evidence
for the prepattern/cooption model of vertebrate jaw
Proc. R. Soc. B (2012)
26
27
28
29
30
31
32
33
34
35
J. A. Finarelli & M. I. Coates
779
evolution. Proc. Natl Acad. Sci. USA 107, 17 262 –17
267. (doi:10.1073/pnas.1009304107)
Didier, D., Stahl, B. J. & Zangerl, R. 1994 Development
and growth of compound tooth plates in Callorhinchus
milli (Chondrichthyes, Holocephali). J. Morphol. 222,
73– 89. (doi:10.1002/jmor.1052220108)
Kemp, A. 1979 The histology of tooth formation in the
Australian lungfish, Neoceratodus forsteri Krefft.
Zool. J. Linn. Soc. 66, 251 –287. (doi:10.1111/j.10963642.1979.tb01910.x)
Reisz, R. R. & Smith, M. M. 2001 Developmental
biology: lungfish dental pattern conserved for 360 Myr.
Nature 411, 548. (doi:10.1038/35079187)
Smith, M. M. & Johanson, Z. 2003 Separate evolutionary
origins of teeth from evidence in fossil jawed vertebrates.
Science 299, 1235–1236. (doi:10.1126/science.1079623)
Carroll, R. L. 1988 Vertebrate paleontology and evolution.
New York, NY: W.W. Freedman and Co.
Grogan, E. D. & Lund, R. 2000 Debeerius ellefseni (Fam.
Nov., Gen. Nov., Spec. Nov.), an autodiastylic chondrichthyan from the Mississippian Bear Gulch Limestone
of Montana (USA), the relationships of the Chondrichthyes, and comments on gnathostome evolution.
J. Morphol. 243, 219–245. (doi:10.1002/(SICI)10974687(200003)243:3,219::AID-JMOR1.3.0.CO;2-1)
Ginter, M. & Sun, Y. 2007 Chondrichthyan remains
from the Lower Carboniferous of Muhua, southern
China. Acta Palaeontol. Pol. 52, 705 –727.
Finarelli, J. A. & Clyde, W. C. 2004 Reassessing hominoid
phylogeny: evaluating congruence in the morphologic and
temporal data. Paleobiology 30, 614–651. (doi:10.1666/
0094-8373(2004)030,0614:RHPECI.2.0.CO;2)
Naylor, G. J. & Adams, D. C. 2001 Are the fossil data really
at odds with the molecular data? Morphological evidence
for Cetartiodactyla phylogeny reexamined. Syst. Biol. 50,
444–453. (doi:10.1080/106351501300318030)
Sallan, L. C. & Coates, M. I. 2010 End-Devonian
extinction and a bottleneck in the early evolution of
modern jawed vertebrates. Proc. Natl Acad. Sci. USA
107, 10 131 –10 135. (doi:10.1073/pnas.0914000107)