Zo01.y. Linn. SOC.,49, p p . 235-245. With 55gures
August 1970
The systematic position of the Triassic ornithischian dinosaur
Lycorhinus angustidens
RICHARD A. THULBORN
Department of Zoology, University College, London, W.C.1*
Accepted for publication November 1969
A topotype of the poorly known reptile Lycorhinus angustidens Haughton (1924) is described.
This specimen demonstrates that Lycorhinus is a genuine ornithischian dinosaur of late
Triassic age and not, as previously supposed, a therapsid. The generic term ‘Heterodontosuum’ Crompton & Charig (1962) is regarded as an invalid junior synonym for Lycorhinus
and is discarded-though the species tuck (now referred to Lycorhinus) is retained as distinct
from angustidens on the basis of slight differences in tooth structure. Lycothinus is included
within the family Hypsilophodontidae of the suborder Omithopoda.
CONTENTS
.
.
.
.
.
.
.
Introduction .
Description
.
Skull bones .
.
Dentition
.
Discussion
.
Acknowledgements .
References
.
PAGE
235
236
236
239
241
244
244
INTRODUCTION
The genus Lycorhinus was established by Haughton (1924) on the basis of a
fragment of mandible bearing eleven cheek teeth and a very conspicuous anterior
‘canine’. The holotype was collected from the Red Beds of the Stormberg Series
at Paballong, near Mount Fletcher, in the Herschel District of Cape Province, South
Africa. The Red Beds are generally acknowledged to be of late Triassic age.
Haughton regarded Lycorhinus as a mammal-like reptile, and there are, indeed, very
marked superficial resemblances between the mandible in Lycorhinus and that in
therapsids such as Cynognathus (Broili & Schroder, 1934). In both cases the jaw is
remarkably deep, there is a prominent canine-like tooth anteriorly, and the cheek
teeth are ornamented with coarse denticles. I n view of these general similarities (and
of the subsequent lack of further specimens) the concept of Lycorhinus as a therapsid
persisted unchallenged until quite recently. Then, in 1962, Crompton and Charig
(in their account of Heterodontosaurus tucki) suggested that Lycorhinus might in fact
be an ornithischian dinosaur. This idea seems to have been prompted by overall
resemblances between the teeth in Lycorhinus and those in the unquestionably
* Present Address: Dept. of Geology, The University, P.O.
235
Box 363, Birmingham 15.
236
R. A. THULBORN
ornithischian Heterodontosaurus. Unfortunately the Lycorhinus holotype did not
provide enough evidence to investigate such a possibility.
This situation renders the specimen described below very important-for it
enables one to attain certain definite conclusions as to the affinities of Lycorhinus.
The new specimen was collected by Dr K. A. Kermack from the Red Beds at
Paballong during the course of an expedition (winter 1960-61) financed by the Royal
Society. This specimen, numbered A.lOO, is preserved in the collection of the Zoology
Department at University College London and has previously been exhibited at the
Geological Society of London (Kermack, 1962). The specimen, from the same locality
as the holotype, is accorded the status of a topotype.
DESCRIPTION
Lycorhinus angustidens Haughton
Haughton, S.H., 1924, pp. 343 to 344, fig. 8.
Holotype. South African Museum catalogue number 3606. Fragment of left
mandible with traces of 12 teeth. From the Triassic Red Beds at Paballong, near
Mount Fletcher, in the Herschel District of Cape Province, South Africa.
Topotype. Number A.lOO. Single block of red sandstone containing pieces of a skull.
Also from the Red Beds at Paballong.
Although the holotype has been lost a latex impression of this is available and
compares well with specimen A.100. Since both of these specimens are from the same
locality there is a slight possibility that they might represent different parts of a single
individual, though this seems unlikely in view of the great hiatus between their two
dates of collection.
Despite the fragmented nature of specimen A100 several important structural
features are readily apparent-notably the antorbital vacuity and the lateral and
superior temporal openings (Figs 1 and 3).
Skull bones
Premaxilla (Figs 1 & 2)
The right premaxilla lacks its dorsal and anterior margins and is exposed in medial
aspect. This bone is 20 mm long and bears three teeth which are bordered lingually
by a bony shelf representing the palatal surface. The premaxilla is quite deep,
measuring at least 9 mm between the external naris and the tooth row. The teeth
are grouped at the rear of the premaxilla (which is edentulous anteriorly). Behind
the third tooth the ventral margin of the bone is deeply excavated in the form of
a wide arch-like diastema.
Maxilla (Figs 1 & 3)
The almost complete right maxilla is exposed in lateral aspect whilst the posterior
part of the left maxilla is seen in medial view. The dentigerous part of the right
maxilla is 47 mm long. It contains eight teeth (or parts of teeth) and has alveoli for
another five. This portion of the maxilla is some 7.5 mm deep and bears a prominent
longitudinal ridge on its lateral surface-this producing, in effect, a distinct recess
T H E ORNITHISCHIAN DINOSAUR L YCORHINUS
237
FIGURE
1. Lycorhinus angustidem. Two views of specimen A.lOO. Matrix indicated by
stippling.
F, Left frontal ; JL, left jugal ; JR, right jugal ; LTF, lateral temporal opening (left side) ;
MDR, right mandible; MXL, left maxilla; MXR, right maxilla; N, left nasal; PMXR,
right premaxilla ; PO, left postorbital ; STF, upper temporal opening (left side).
immediately above the tooth row. At its anterior end the maxilla is slightly inflated
and is produced postero-dorsally as a conspicuous ascending process. This defines
the antero-ventral angle of the large triangular antorbital vacuity.
Jugal (Fig. 1)
T h e jugal is a tri-radiate bone, comprising dorsal, anterior and posterior processes.
T h e posterior ramus, longest and narrowest of the three, defines the ventral limit
3
I
I cm
I
FIGURE2. Lycorhimrs angustidens. fight premaxilla from specimen A. 100. Medial view.
Teeth numbered from the front.
D, Diastema behind third tooth.
R. A. THULBORN
238
of the lateral temporal opening and is peculiar in being strongly up-curved at its
posterior end (though this may be due to post-mortem distortion). This ramus is
damaged-so that the nature of its junction with the quadrato-jugal (not preserved)
cannot be determined. The dorsal branch of the jugal is broad and short and is joined
above to the postorbital bone, though the intervening suture has been entirely
obliterated by fissuring. The anterior jugal ramus is intermediate between the other
two, both in thickness and in length. It tapers to the front, the inferior margin sloping
upwards to meet the almost horizontal dorsal edge. The slanting ventral edge represents the sutural contact with the maxilla-which is prolonged posteriorly to underlie
the anterior part of the jugal.
Icm
aQ
....
.:.....
..:,. .
.*
.*.,.5&.::*:*
.....*:..-.--.*.
......
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.:.a
FIGURE
3. Lycorhimrs angutidens. Right maxilla from specimen A.lOO. Lateral view. Matrix
indicated by regular stippling, broken surfaces by diagonal shading.
AOV, Antorbital vacuity; D, diastema; N, medial surface of left nasal.
Postorbital (Fig. 1)
The postorbital, represented only on the left side, is drawn out posteriorly into
a long and acutely pointed process. The inferior edge of this process forms the dorsal
limit of the lateral temporal opening. The dorsal edge of the same process probably
met an antero-lateral extension of the squamosal (not preserved) so as to form a bony
bar between the lateral and superior temporal openings. The postorbital is joined
anteriorly to the frontal bone, though the intervening suture is not discernible.
Frontal (Fig. 1)
The left frontal, though poorly preserved, is distinctly longer than wide and is
gently arched in a dorsal direction. Its thin, sharp and fairly straight medial edge
T H E ORNITHISCHIAN DINOSAUR LYCORHINUS
239
indicates a simple edge-to-edge suture between left and right frontals. T h e other
edges are damaged and yield no useful information.
Nasal (Fig. 1)
T h e medial surface of the very poorly preserved left nasal is visible within the
right antorbital vacuity. None of its edges is preserved and the form and extent of
the bone cannot be determined.
Mandible (Figs 1 and 5 )
T h e anterior part of the right mandible, bearing seven teeth (or parts of teeth),
is exposed in lateral aspect. This jaw fragment is composed entirely of the dentary
bone and is remarkably deep (nearly 20 mm)-though this may again have been
emphasised by crushing. T h e lateral surface of the jaw is gently convex in a vertical
direction whilst its ventral margin is thick and broadly rounded. Anteriorly, where
it shows a slight decrease in height, the jaw terminates in an oblique fracture surface,
there being no trace preserved of any predentary bone.
Dentition
Premaxillary teeth (Figs 1 and 2)
T h e first two premaxillary crowns are smooth, slender and acutely pointed conical
pegs which are directed ventrally and slightly to the front. T h e first crown is damaged
whilst the second, which is slightly larger, is 6.5 mm tall and has a maximum mesiodistal breadth of 2 mm. T h e lingual surface is convex from mesial to distal edgesexcept near the occlusal tip of the crown, where there is a broad and flat wear
facet. Similar wear is evident on the third premaxillary tooth.
T h e third premaxillary tooth is easily distinguished from the preceding two on
account of its much greater size and its caniniform appearance. This acutely pointed
crown is directed ventrally and slightly to the rear. It is 10.5 mm high and has a
maximum mesio-distal width of 4 mm. Its mesial edge, though it is slightly damaged,
seems to be simple and devoid of ornament. ISear the alveolus the distal margin is
thick and rounded. Towards the occlusal end this distal edge is thinner and bears
a sharp ridge which is finely serrated. T h e minute rounded denticles responsible for
this serration are very closely grouped (about five per millimetre).
Maxillary teeth (Figs 1, 3 and 4)
T h e right maxilla bears traces of 13 teeth though only three crowns (7th, 8th and
9th from the front) are at all well preserved. These crowns, which are worn flat at
their occlusal ends, are exposed in labial aspect and are about 4 mm tall. Each crown
is narrowest (mesio-distally) at the alveolar margin (2 mm) and is widest at the occlusal
end (3 mm). The labial crown surface is demarcated from the root by a faint swelling,
though this varies in strength from tooth to tooth. T h e distal margin is produced
labially as a thin and well rounded ridge which is bordered towards the mid-line
of the crown by a broad and shallow groove. This distal ridge is the most prominent
R. A. THULBORN
240
single feature upon the labial surface. The mesial margin is basically similar-though
the ridge and groove are nowhere as obvious as on the distal edge. The labial surface
is further ornamented with a weak median ridge-this being flanked, at the worn
occlusal end, by vestiges of one or two minor ribs.
The teeth in the posterior part of the left maxilla are exposed in lingual aspect
(Fig. 4) and are, like those of the right side, worn flat at their occlusal ends. A faint
swelling is developed where the lingual crown surface merges with the root. The
lingual surface bears an ill-defined median rib, often showing slight deflection in a
mesial direction, which is flanked by broad and shallow grooves. The distal edge
is produced, as on the labial side, into a thin ridge.
I
I
FIGURE 4. Lycorhimrs angustidens. The teeth in the left maxilla of specimen A.lOO. Lingua
aspect Arrow indicates anterior.
Mandibular teeth (Figs 1 and 5 )
The first seven teeth in the right mandible are exposed in labial aspect. The first
mandibular tooth is by far the largest in the entire dentition of Lycorhinus angustidens,
being generally similar to (but considerably larger than) the third premaxillary tooth.
It has a height of 17 mm, is acutely pointed, recurved and somewhat flattened from
the labial side; its maximum mesio-distal width is 5.5 mm. The mesial margin is
thick (2.5 mm) and flattened near the alveolar border. Towards the occlusal tip of
the crown the mesial edge becomes much narrower and is ornamented with a thin
and sharp vertical ridge which is very finely serrated. The distal margin of the tooth
is simple and bears no trace of comparable ornament.
Behind this mandibular caniniform tooth there is a short edentulous gap followed
by a series of smaller teeth. These cheek teeth increase in size posteriorly and are
generally comparable in appearance to those in the maxilla. These mandibular teeth
are, however, unworn. Each crown expands in mesio-distal width towards its occlusal
end and bears, at the junction with the root, a very faint swelling. On the labial
surface there is a weak median rib flanked by shallow grooves whilst the distal edge
is produced as a thin and rounded ridge. The median rib terminates in a broadly
triangular and salient cusp. The thickened distal margin terminates in a narrow cusp
which is separated from the median one by two (sometimes three) small denticles.
Similar denticles occur mesial to the median cusp. These marginal denticles are all
of similar size and are slightly divergent rather than parallel. They are prolonged
on to the labial surface as very minor ridges and are not so well marked on the smaller
crowns at the front of the mandible.
T H E ORNITHISCHIAN DINOSAUR LYCORHINUS
241
5 rnm
FIGURE
5 . Lycorhinus angustidens. Anterior part of right mandible from specimen A.lOO.
Lateral view. Matrix and broken surfaces shown as in Fig. 3.
DISCUSSION
Specimen A.lOO exhibits many points of similarity with the holotype of Lycorhinus
angustidens (bearing in mind that comparisons must, of necessity, be limited to the
anterior part of the mandible and its teeth). I n both cases the jaw is exceptionally
deep. In its pronounced depth the mandible resembles that in cynodonts (e.g.
Cynognathus) rather than that in known Triassic ornithischians (Fabrosaurus,
Ginsburg 1964 ; Tutisaurus, Simmons 1965 ;Pisanosuuyus, Casamiquela 1967). I n both
specimens the extremely characteristic caniniform tooth is present at the front of the
lower jaw. I n the holotype this tooth is 19.5 m m tall, with a maximum mesio-distal
width of 7 m m ; in specimen A.lOO the comparable dimensions are 17 and 5.5 mm.
Further, the caniniform teeth in these two examples bear precisely similar
ornament-the mesial edge being finely serrated whilst the distal edge is simple.
I n both cases there is a slight, but distinct, gap between the caniniform and the most
anterior cheek tooth.
17
242
R. A. THULBORN
Further comparisons are difficult for two reasons: (a) the holotype was exposed
and described only in lingual aspect (Haughton, 1924), and (b) the cheek teeth of
the holotype are worn (despite Haughton’s assertion to the contrary). Comparisons
are, however, possible between the teeth in the holotype and those in the left maxilla
of specimen A.lOO (these latter being exposed from the lingual side and showing
definite traces of wear). These worn crowns (Fig, 4) are virtually identical with those
described and figured by Haughton (1924). In each case the lingual crown surface
bears a broad median rib and the distal margin is produced as a thin and erect ridge,
the mesial edge showing only a faint tendency towards elaboration into a similar ridge.
These detailed similarities with the holotype leave little doubt that specimen A. 100
does represent Lycorhinus angustidens. Having established the identity of this specimen
it is now possible to re-examine the affinities of the genus Lycorhinus.
Specimen A.lOO, imperfect though it is, considerably amplifies our knowledge of
Lycorhinus. This specimen possesses structural features of unmistakable archosaurian
type-principally the large antorbital vacuity and the presence of both lateral and
superior temporal openings. Haughton’s theory of therapsid affinities is immediately
broken down when one considers the diapsid construction of the Lycorhinus skull.
Of all adequately known Triassic archosaurs Lycorhinus resembles most closely the
ornithischian dinosaur Heterodontosaurus tzccki (Crompton & Charig, 1962). The
cranial and dental peculiarities of Lycorhinus can be matched with very similar features
in Heterodontosaurus. Firstly, both animals possess a large, triangular and widely-open
antorbital vacuity. In both cases the lateral surface of the maxilla is ‘stepped’, being
distinctly recessed above the tooth row. This recess is rather more pronounced in
Heterodontosaurus than it is in Lycorhinus, though this slight distinction might well
be attributed to differences in preservation (specimen A.lOO having been crushed from
above whilst the Heterodontosaurus holotype has suffered lateral compression).
Prominent caniniform teeth are characteristic of both genera. I n each case the
mandibular caniniform is distinctly larger than that at the rear of the premaxilla.
In Lycorhinus the mesial edge of the lower caniniform is serrated ;Crompton & Charig
(1962) maintain that in Heterodontosaurus the distal edge of the lower caniniform
is serrated. Very careful preparation of this tooth in specimen A.lOO has confirmed
that the mesial edge alone is serrated in Lycorhinus. Dr Charig has pointed out (pers.
comm.), after further preparation of the Heterodontosaurus holotype, that both mesial
and distal edges of the lower caniniform are serrated in this animal. This,then, is
a genuine distinction-in Heterodontosaurus both edges of the lower caniniform are
serrated ; in Lycorhinus the mesial edge is serrated but the distal edge is simple.
Both forms are similar in the number and appearance of the premaxillary teeth.
In each case the first two teeth are simple conical pegs whilst the third is much larger
and canine-like (though in neither case as large as the mandibular caniniform). In
Lycorhinus angustidens the distal edge of the upper caniniform bears a row of minute
denticles ; close examination of the Heterodontosaurus holotype has revealed the
existence of identical ornament. The two animals are comparable, moreover, in terms
of distribution of teeth within the premaxilla-in each case the three teeth are grouped
at the rear of the bone and are preceded by a wide edentulous zone. The Lycorhinus
premaxilla is also remarkably deep below the external naris, this feature being quoted
'I'HE ORNITHISCHIAN DINOSAUR L YCORHINUS
243
as diagnostic of the genus Heterodontosaurus by Crompton & Charig (1962). Lycorhinus further resembles Heterodontosaurus in possessing a wide and deep diastema
between premaxilla and maxilla, a feature which may be regarded as quite exceptional
amongst archosaurs.
T h e Lycorhinus cheek teeth are basically similar in appearance to those of Heterodontosaurus, though there are some minor distinctions. Heterodontosaums has 12 teeth
in the maxilla, Lycorhinus has 13. I n both cases the largest crowns are found near
the middle of the tooth row. T h e unworn cheek teeth of Lycorhinus are completely
enamelled. The heavily worn crowns show that this enamel coating is rather thin
and that it may be lost from the more exposed portions of the teeth. I n the holotype
of Heterodontosaurus tucki the teeth are only partially enamelled, though this may
be immediately reconciled with the fact that these teeth are much abraded. The cheek
teeth of the Heterodontosaurus holotype are planed off at their occlusal ends and have
obviously suffered prolonged wear. A second specimen of Heterodontosaurus (a piece
of maxilla in the care of Dr A. J. Charig, British Museum (Natural History)) supports
this conclusion. The unworn teeth in this specimen (as yet undescribed) bear marginal
denticles which are in every way comparable with those in the cheek teeth of
Lycorhinus. I n each case there is a conspicuous median denticle which is confluent
with a broad rib on the crown surface. This median rib is flanked by shallow vertical
grooves whilst the mesial and distal crown margins are produced into thin and sharp
ridges. Lycorhinus and Heterodontosaurus are also identical in the mode of tooth wear :
the lingual sides of the maxillary crowns and the labial sides of the mandibular crowns
are planed off to produce sharp and chisel-like occlusal ends. T h e cheek teeth of
the two animals may, in fact, be distinguished only with some difficulty, the main
differences concerning not the type of tooth ornament but rather its relative strength
of expression. The teeth of Lycorhinus bear on their labial surfaces median ribs which
are neither as thin nor as sharp as those in Heterodontosaurus; the excavated regions
flanking this median rib are somewhat more sharply defined in Heterodontosaurus.
T h e significance of these resemblances and differences may now be examined. The
pronounced resemblances with Heterodontosaurus indicate that Lycorhinus must be
considered a genuine ornithischian dinosaur. Though the mandibular symphysis is
lacking from both known specimens of Lycorhinus there is slight evidence that this
was formed by a predentary bone, diagnostic of ornithischian dinosaurs. First, both
known specimens have the mandible broken across obliquely at the front. The
similarity of the breakage affecting the two specimens certainly implies that there
was some plane of weakness in this place-as might have been produced by a suture
between dentary and predentary. Second, the premaxillary teeth show wear facets,
on their lingual surfaces, which might well have resulted from their working against
a horn-sheathed predentary at the mandibular symphysis.
T h e obvious similarities between Heterodontosaurus and Lycorhinus also indicate
that the two animals are closely related.
Some of the structural peculiarities noted above seem to be restricted amongst early
ornithischians to these two genera alone. Obviously one might cite the occurrence
of prominent caniniform teeth; such enlarged teeth are not encountered in any other
Triassic or later ornithischian-with the possible exception of Geranosaurus atavus
244
R. A. THULBORN
(Broom, 1911). The same is true of the diastema between premaxilla and maxilla.
Features such as these emphasise the closeness of Lycorhinus to Heterodontosaurus
and at the same time accentuate their incompatibility with approximately coeval
ornithischians-Fabrosaurus (Ginsburg, 1964), Tatisaurus (Simmons, 1965) and
Pisanosaurus (Casamiquela, 1967).
Heterodontosaurus and Lycorhinus may be separated by only a few and relatively
minor structural differences-principally concerning the ornament of the teeth. In
view of the striking resemblances between the two it is here suggested that the generic
term Heterodontosaurus should be regarded as an invalid junior synonym for Lycorhinus
(which has priority by almost 40 years). The species tucki, now referred to Lycorhinus,
is retained as distinct from angustidens for two reasons :(1) There are minor distinctions in the teeth. The species tucki has both mesial
and distal edges of the lower caniniform finely serrated ;in angustidens only the mesial
edge is so ornamented. Fluting on the labial surfaces of the cheek teeth is more
pronounced in the species tucki.
(2) There appears to be a slight stratigraphic disparity. Lycorhinus angustidens is
from the Red Beds whilst Lycorhinus (‘Heterodontosaurus’)tucki comes from the Cave
Sandstone. In terms of classic stratigraphy the Red Beds formation is regarded as
antecedent to the Cave Sandstone-though in all probability the intervening boundary
is diachronous. Further, Dr Kermack suggests (pers. comm.) that this stratigraphic
distinction may be even less significant, indicating the provenance of specimen A. 100
as ‘. .. very high up in the succession ... within, or at least at the base of, the Cave
Sandstone.’
Whilst there are definite differences between Lycorhinus angustidens and L.
(Heterodontosaurus) tucki these serve only to distinguish the animals at species level
and do not warrant their separation into distinct genera. T o return to the status of
Lycorhinus-the evidence seems quite incontrovertible. This reptile should evidently
be placed alongside forms such as Hypdophodon and Thescelosaurus within the family
Hypsilophodontidae of the suborder Ornithopoda. This category is reserved for
ornithischian dinosaurs of primitive aspect which are characterised by the possession,
amongst other features, of teeth in the premaxilla.
ACKNOWLEDGEMENTS
My particular thanks go to Dr K. A. Kermack for permitting me to work upon
the specimen described above and for much useful advice and constructive criticism.
I am also indebted to Dr A. J. Charig, of the British Museum (Natural History) for
much information on Heterodontosaurus. This work was financed by a research
studentship from the Natural Environment Research Council. Miss S. J. Plummer
has kindly read and criticised the manuscript.
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