The intramandibular joint in squamates, and the phylogenetic relationships of the fossil snake Pachyrhachis problematicus Haas

80 Pages·2000·4.6 MB·English

by  Rieppel, Olivier

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FIELDIANA
WDlOOrifBWBy
Geology
NEW
SERIES, NO. 43
The Intramandibular Joint in Squamates, and
the Phylogenetic Relationships of the Fossil
Snake Haas
Pachyrhachis problematicus
Olivier Rieppel
Hussam Zaher
March 31, 2000
Publication 1507
PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY
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Croat,T B. 1978. FloraofBarroColorado Island. StanfordUniversityPress, Stanford, Calif, 943 pp.
Grubb, P. J., J. R. Lloyd, and T. D. Pennington. 1963. A comparison ofmontane and lowland rain forest in
Ecuador. I.The foreststructure,physiognomy, andfloristics. Journal ofEcology, 51: 567-601. D
Langdon, E. J. M. 1979. Yage among the Siona: Cultural patterns in visions, pp. 63-80. I_n Browman, L.,
andR.A. Schwarz, eds., Spirits, Shamans, and Stars. Mouton Publishers,The Hague,Netherlands.
Murra, J. 1946. The historic tribes of Ecuador, pp. 785-821. In Steward, J. H., ed., Handbook of South
American Indians. Vol. 2, The Andean Civilizations. Bulletin 143, Bureau of American Ethnology,
SmitGhsonianInstitution,Washington, DC.
Stolze, R. 1981. Ferns and fern allies ofGuatemala. Part II. Polypodiaceae. Fieldiana: Botany, n.s., 6: 1-
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UBRAIW
OEOUOGf
FIELDIANA
Geology
NEW
SERIES, NO. 43
The Intramandibular Joint in Squamates,
and the Phylogenetic Relationships of the
Fossil Snake Pachyrhachis problematicus Haas
Olivier Rieppel
Department ofGeology
FieldMuseum ofNatural History
1400 South Lake Shore Drive
Chicago, Illinois 60605-2496
U.S.A.
Hussam Zaher
Departamento de Zoologia
Instituto de Biociencias
Universidade de Sao Paulo
Caixa Postal 11461
05422-970 Sao Paulo, SP
Brasil
Accepted July 28, 1999
Published March 31, 2000
Publication 1507
PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY
© 2000 Field Museum of Natural History
ISSN 0096-2651
PRINTED IN THE UNITED STATES OF AMERICA
Table of Contents 3. Lowerjaw ofLanthanotus borneensis 6
4. Lowerjaw ofPlatecarpus 7
5. Lowerjaw ofAnilius scytale 11
Abstract 1 6. Transverse section through the lower
Introduction 1 jaw ofAnilius scytale 12
Materials and Methods 2 7. Lowerjaw of Cylindrophis ruffus 12
The Intramandibular Joint in Squamates 3 8. Lowerjaw of Cylindrophis maculatus 14
The Intramandibular Joint in Varanus 9. Lowerjaw ofMelanophidium puncta-
and Lanthanotus 3 tus, Platyplecturus madurensis, and
The Intramandibular Joint in Mosasaurs .... 7 Pseudotyphlops philippinus 16
The Compound Bone of the Ophidian 10. Lowerjaw of Plecturus perroteti 17
Mandible 8 11. Lowerjaw of Python reticulatus 19
The Intramandibular Joint in Scolecophi- 12. Lowerjaw of Lichanura trivirgata ro-
dians 9 seofusca 21
The Intramandibular Joint in Anilioids 10 13. Lowerjaw of Calabaria reinhardti and
The Intramandibular Joint in Basal Ma- Charina bottae 22
crostomatans 18 14. Lowerjaw ofBoa constrictor impera-
The Skull and Lower Jaw of Pachy tor 23
rhachis 24 15. Skull ofPachyrhachisproblematicus .... 26
Character Evidence for the Monophyly 16. Lowerjaw of Pachyrhachis problema-
of the Pythonomorpha 29 ticus 28
Cladistic Analysis 49 17. Snake interrelationships 61
The Phylogenetic Relationships ofPachy
RHACHIS, DlNILYSlA, AND DlBAMUS 61
Discussion: Snake Origins, and Homolo-
gy Versus Convergence 62 List of Tables
Notes Added in Proof 65
Acknowledgments 66
Literature Cited 66 Data matrix for the analysis of squamate
50
interrelationships
List of Illustrations
1. Lowerjaw of Varanus 4
2. Coronoid and splenial of Varanus 5

The Intramandibular Joint in Squamates, and the
Phylogenetic Relationships of the Fossil Snake
Haas
Pachyrhachis problematicus
Olivier Rieppel Hussam Zaher
Abstract
A review of the morphology of the lowerjaw in varanoid squamates, including mosasaurs,
and basal snakes (scolecophidians, anilioids, basal macrostomatans) reveals a greaterdegree of
variability in the differentiation ofthe intramandibularjoint than had previously been recorded.
In particular, the mandibularjoint ofmosasauroid squamates and snakes differs fundamentally.
In mosasaurs, the dentary is primarily suspended from the prearticular and the posteriorly
concave splenial receives the anteriorly convex angular. In snakes, the dentary is primarily
suspended from the surangular portion ofthe compound bone, and the angular is the receiving
part in the mobile contact with the splenial. Characters ofthe intramandibularjoint, along with
those resulting from a review of the cranial anatomy of the fossil snake Pachyrhachis from
the basal Upper Cretaceous ofEin Jabrud, are used in a review ofsquamate interrelationships.
The results corroborate macrostomatan affinities of Pachyrhachis and do not support the hy-
pothesis that snakes originated from mosasauroids, a clade ofmarine varanoid squamates from
the Cretaceous.
Introduction affinities of the Pythonomorpha were established
by Cope (1869)onthebasisofsimilaritiesofden-
Mosasauroids are aclade offossil marine squa- tition, the suspension ofthe lowerjaw, and intra-
mates related to extant monitor lizards. Theirear- mandibular kinetics.
liest fossil occurrence is in shallow marine de- More recently, cladistic supporthas been build-
posits of early Cenomanian age (lower Upper ing in support of a monophyletic clade Pythono-
Cretaceous) of southern Europe. These stem- morpha that would include platynotan (varanoid)
group taxa, variously referred to as Aigialosauri- squamates and mosasauroids as well as snakes
dae and/or Dolichosauridae, remain relatively (Lee, 1997). Configuration of such a clade has
poorly known compared to later members of the been corroborated by the redescription ofa fossil
clade, the Mosasauridae. The crown-group mo- snake with hind limbs from the basal Upper Cre-
&
sasaurs adopted fully pelagic habits and include taceous of the Middle East (Caldwell Lee,
&
species that were among the largest predators of 1997; Lee Caldwell, 1998). Originally de-
the late Cretaceous seas. Mosasaurs became ex- scribed by Haas (1979, 1980), the status of this
tinct at the close of the Cretaceous. fossil snake taxon, Pachyrhachis problematicus,
The intramandibularjoint has played a promi- remains problematic. Although already consid-
nent role in discussions of mosasauroid relation- ered by some to be the ideal fossil link between
ships with snakes ever since Cope (1869) com- snakes and mosasauroids (Carroll, 1988), it was
mented on the ophidian affinities ofhis order Py- also noted that those characters that are snakelike
thonomorpha. In the Pythonomorpha, Cope in Pachyrhachis resemble relatively advanced
(1869) included two families of mosasaurs, the (macrostomatan) snakes instead of more basal
Clidastidae and the Mosasauridae. The ophidian members of the group (Haas, 1979, 1980; Riep-
FTELDIANA: GEOLOGY, N.S., NO. 43, MARCH 31, 2000, PP. 1-69
pel, 1994). This controversy is still alive, as a re- ((Snake + dibamid) amphisbaenian) clade (Reed-
analysis of the cladistic relationships of Pachy- er, 1995).
rhachis showed it to be the sister taxon of ma- As is true for every phylogenetic analysis, hy-
crostomatan snakes rather than a primitive snake potheses ofrelative relationships are only as good
providing a link between this group and mosa- as the characterevidence they are basedon (Riep-
& We
saurs (Zaher, 1998). pel Zaher, in press). propose to review, in
Considering Pachyrhachis as the most primi- this study, the lower jaw anatomy of varanoid
tive snake and "an excellent example of a tran- squamates and snakes in detail, bearing in mind
sitional taxon" (Scanlon et al., 1999) between that superficial and potentially misleading resem-
mosasauroids and snakes (Lee, 1998) has impor- blances can result from two factors. One is that
tant consequences, as this pattern ofrelationships increased mobility in the lower jaw, as much as
suggests that snakes had a marine rather than ter- increased cranial kinesis in general, results from
restrial (fossorial) origin. Shared derived charac- areduction inboneoverlap, whichinturn is likely
ters that have been used in support of a mono- to result from paedomorphosis (assuming the aki-
phyletic Pythonomorpha recall Cope's (1869) netic condition to be plesiomorphic; Irish, 1989).
analysis and were derived from braincase mor- The otherfactorresults from structural constraints
phology and its relation tojaw suspension, lower in the development of an intramandibular joint.
jaw anatomy, and characters ofthe dentition (Lee, As Gauthier (1982, p. 46; see also Underwood,
1997; Lee & Caldwell, 1998). We havepreviously 1957, p. 25) pointed out, "some similarity is to
critically assessed the characters derived in these be expected, especially since there is but one
latter studies from squamate tooth implantation place in a squam—ate mandible where a mobile
and replacement (Zaher & Rieppel, 1999) and joint could form between the dentary-splenial
from braincase morphology and its relation tojaw and the postdentary bones." This point is partic-
suspension (Rieppel & Zaher, in press). The intra- ularly well borne out by comparison with the con-
mandibularjoint has traditionally been an impor- vergently differentiated intramandibular joint in
t(DaoCnwatemclphl,araa1nc9td2e3rB)o.igneIdrnitstc(hu1esi9sr5i4oc)nlsacsoosmficpsinmlaoekndeoargerllaaaprtgiheo,nnsMhuicmp--s Hgweearsyipscehr,osrh1no9i7ws3,)a.aTrhfeodesuscislepldebnipiraodls,t(eGfrorireorgeoxreayxmt,peln1te9,5i1nw;iltlGhioansl-e-
bvearraonfoicdh,arraecltaetriosnsinhispuopfposrntakoefs,anamaongnugimwohripchh,toher jetotaixnaatl.t,ihna1t9td8h8ee)v.eanlaoRlpeyfsaeirnseinoncftersatnmoaakntedhiebreuillanattrirojanomsiahnnitdpis(bEuswltiaelslr
oifntrthaemacnhdairbaucltaerrsjoeinntumfeirgaurteeddpbryomiMnceDnotlwye.llMaannyd tohrermeefroreerheadvuecttioontracnhsarcaecntderssupeirnfiocriadlersitmoilarreivteiaels
Bogert (1954) came under severe criticism (Un- details of morphology. The characters of Lee
derwood, 1957), but anguimorph, or varanoid, re- (1997) will, in the following, be referenced as
lationships of snakes continued to be discussed L97; the character evidence of Lee and Caldwell
(McDowell 1972; Schwenk, 1988; see also Riep- (1998) will be referenced as LC98.
pel, 1988, for a review). Interestingly, the first
large-scale cladistic analysis of squamate interre-
lationships (Estes et al., 1988) did not provide
strong support for anguimorph, or varanoid, re- Materials and Methods
lationships of snakes, which in this study were
classified as Scleroglossa (all non-iguanian squa- The specimens examined for this study are list-
mates) incertae sedis. However, parsimony anal- ed below. Institutional abbreviations are bmnh,
ysis of this data set put snakes close to fossorial British Museum (Natural History); fmnh, Field
or burrowing squamates such as dibamids and Museum of Natural History; HUJ-Pal., Paleonto-
amphisbaenians (see also Rage, 1982). Whereas logicai Collections, Hebrew University, Jerusa-
this latter hypothesis has recently gained further lem. Drawings were made with a Wild binocular
support from morphological evidence (Haller- M-8 equipped with a camera lucida.
mann, 1998), molecular data support anguimorph
relationships for snakes (Forstner et al., 1995; Anilius scytale, fmnh 11175, 35688, uncata-
Reeder, 1995). An as yet unpublished total evi- logued
dence approach, combining molecular (DNA) and Boa constrictor imperator, fmnh 22353, 22363
morphological data, unambiguously supported a Calabaria reinhardti, fmnh 31372
FIELDIANA: GEOLOGY
Charina bottae, fmnh 31300 splenial and the postdentary bones are more com-
Cylindrophis ruffus, fmnh 13100, 131780 plex in medial view of the tooth-bearing shelf of
Cylindrophis maculatus, bmnh 1930.5.8.48, un- the mandible of Varanus. The anteriortooth-bear-
catalogued ing part of the dentary forms a gentle slope (a
Lanthanotus borneensis, fmnh 74711 discrete subdental shelf is absent; L97: char. 67),
Leptotyphlops emini, fmnh 56374 which in front of the splenial projects ventrally,
Lichanura trivirgata roseofusca, fmnh 8043 thus overhanging Meckel's canal in medial view.
Melanophidium punctatus, bmnh 1930.5.8.119 The anterior part of Meckel's groove opens ven-
Pachyrhachisproblematicus, HUJ-Pal. 3659 trally relative to the sagittal plane ofthe mandib-
Platecarpus sp., fmnh UC 600 ular ramus, as it is defined by the lateral wall of
Platyplecturus madurensis, bmnh 1930.5.8.111 the dentary and the medial tooth-bearing shelf
Plecturus perroteti, bmnh 1930.5.8.105 (L97: char. 69; LC98: char. B13). Theanteriorend
Pseudotyphlopsphilippinus, bmnh 1978.1092 of Meckel's groove opens medioventrally in the
Python reticulatus, fmnh 31281, 31329 live animal because the lower jaw is rotated
Typhlops sp., fmnh 98952 around its long axis in such a way as to bring the
Varanus komodoensis, fmnh 22199 tooth row into an upright position and to expose
Varanus sp., fmnh 195576 Meckel's cartilage medioventrally for the inser-
Xenopeltis unicolor, fmnh 11524 tion of anterior intramandibular muscles. More
posteriorly, the tooth-bearing shelf merges into
the septum that separates Meckel's canal from the
more dorsolaterally positioned canal for the al-
The Intramandibular Joint in veolar ramus of the mandibular division of the
Squamates trigeminal nerve. The ventral part ofthe posterior
margin of this intramandibular septum is deeply
The Intramandibular Joint in Varanus and concave. Its concavity defines the dorsal and an-
Lanthanotus terior margin of the anterior inferior alveolar fo-
ramen, the posterior and ventral margin of which
Among extant varanoids {Heloderma, Lantha- is defined by the splenial as it contacts the medial
notus, and Varanus), the intramandibular joint surface of the dentary. Above the posterior con-
shows various degrees ofdifferentiation, least de- cavity ofthe septum, the dentary is broadly over-
veloped in Heloderma, most developed in Lan- lapped by the splenial in medial view. Dorsal to
thanotus. the dentary-splenial overlap, the dentary forms a
In Varanus (Fig. 1), the posterior ends of the very short coronoid process defining a postero-
dentary and of the splenial lie entirely in front of ventral recess (notched in lateral view but not in
the apex of the coronoid process (L97: char. 72). medial view) into which fits the anteriordorsal tip
In lateral view, the posterior end of the dentary of the coronoid.
shows a more or less distinctly developed bicon- The splenial (Fig. 2C) itself is roughly of an
cave posterior margin. A smaller, dorsally located arrowhead shape in medial view. A slender and
concavity or indentation receives the anterior tip pointed posteroventral projection overlaps the an-
of the coronoid. The broad and, in some species, terior end of the angular (L97: char. 73; LC98:
shallow concavity below the coronoid-dentary char. B12). A broad posterior dorsal projection
contact broadly overlaps the anterior end of the provides the medial closure of Meckel's canal at
surangular. The ventral margin of the dentary is the level of the anterior end of the surangular.
drawn out into a short posteriorprocess that over- Posterodorsally, the splenial contacts the anterior
laps with the anteriorendofthe angular. The prin- process of the coronoid (L97: char. 77; LC98:
cipal element on which the dentary is supported char. B14), anterodorsally it contacts the posterior
is the surangular. end of the dentary, and posteriorly it defines the
In lateral view, the splenial and angula/ form a anterior margin of the subcoronoid fossa (L97:
broadly overlapping, obliquely oriented contact in char. 79; LC98: char. Bl 1), below which it over-
Varanus. More precisely, the tapering posterior laps the anterior end of the prearticular. Anteri-
endofthe splenial superficially overlapsthebroad orly, the splenial is drawn out into a tapering pro-
anterior end of the angular and curves around its cess that reaches to about the midpoint oftheden-
ventral margin (L97: char. 74; LC98: char. B12). tary. Along the posterior two thirds of the length
Sutural relations between the dentary and of the splenial, a horizontal shelf projects from
RIEPPEL AND ZAHER: INTRAMANDIBULAR JOINT IN SQUAMATES
VII hy
VII hy
Fig. 1. The lower jaw of Varanus (based on Varanus komodoensis, fmnh 22199). A, lateral view; B, medial
view; C, disarticulated medial view. Notto scale. Abbreviations forall figures: ale, alveolarnervecanal;amf,anterior
mylohyoid foramen; an, angular; ar, articular; c, coronoid; cp, compound bone; d, dentary; mf, mental foramen; pi,
palatine; pmf, posterior mylohyoid foramen; pra, prearticular; sa, surangular; saf, anterior surangular foramen; sp,
splenial; mc (or Mc), Meckel's cartilage; mg (or Mg), Meckel's groove; VII hy, chorda tympani foramen.
FIELDIANA: GEOLOGY