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Full text of "A revision of the Latipinnate ichthyosaurs of the Lower Jurassic of England (Reptilia, Ichthyosauria)"

Life Sciences Contribution 
Royal Ontario Museum 



100 



A Revision of the Latipinnate 
Ichthyosaursof the Lower Jurassic 
of England (Reptilia: Ichthyosauria) 



C. McGowan 



'.«;■ 



i.?* 






Digitized by tine Internet Arciiive 

in 2011 with funding from 

University of Toronto 



http://www.archive.org/details/revisionoflatipiOOnncgo 



LIFE SCIENCES CONTRIBUTIONS 
ROYAL ONTARIO MUSEUM 
NUMBER 100 



c. McGowAN A Revision of the Latipinnate 

Ichthyosaurs of the Lower Jurassic 
of England (ReptiUa: Ichthyosauria) 



Publication date 6 December 1974 

ISBN 0-88854-165-1 

Suggested citation: Life Sci. Contr., R. Ont. Mus. 



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Editor: j. c. barlow 
Associate Editor: a. r. emery 
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CHRISTOPHER MCGOWAN is Associatc Curator, Department of Vertebrate Palaeontology, 
Royal Ontario Museum, and Assistant Professor, Department of Zoology, University 
of Toronto. 



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A Revision of the Latipinnate 
Ichthyosaurs of the Lower Jurassic 
of England (ReptiHa: Ichthyosauria) 



Abstract 

A study of 60 latipinnate ichthyosaurs from the English Lower 
Lias (Lower Jurassic) reveals only four valid species; Ichthyo- 
saurus communis Conybeare, /. breviceps Owen, /. conybeari 
Lydekker, and /. tenuirostris Conybeare. Emended diagnoses 
and descriptions are given. /. communis, a medium-sized 
ichthyosaur reaching a maximum length of about 2.5 m, is the 
commonest species. /. tenuirostris, of similar size to /. com- 
munis, is less common and characterized by extreme snout 
length. /. breviceps, a small species with a maximum recorded 
body length of 1.3 m, is scarce and is characterized by an ab- 
breviated snout. The rarest latipinnate is /. conybeari, with 
only two individuals known. Both are small, with an estimated 
body length of 1.3 m in the larger individual. The snout is 
long, slender, and sharply pointed, and the orbit appears to be 
relatively large. 

Zusammenfassung 

Eine Studie iiber 60 latipinnate Ichthyosaurier aus dem eng- 
lischen unteren Lias (Unterer Jura) ergibt nur vier giiltige 
Arten; Ichthyosaurus communis Conybeare, /. breviceps 
Owen, /. conybeari Lydekker, /. tenuirostris Conybeare. Ver- 
besserte Diagnosen und Beschreibungen werden gegeben. /. 
communis, die gewohnlichste Art, ist ein mittelgrosser Ichthyo- 
saurier, welcher eine Lange von ungefahr 2.5 Meter erreicht. 
/. tenuirostris, von ungefahr derselben Grosse wie /. com- 
munis, ist seltener und zeichnet sich durch seine ungewohnlich 
lange Schnauze aus. /. breviceps, eine kleine Art, mit einer 
Lange bis zu 1.3 Meter, ist sehr selten und zeichnet sich durch 
seine verkiirzte Schnauze aus. Die seltenste Art ist /. conybeari, 
von welcher nur zwei Exemplare bekannt sind. Beide sind 
klein, das grossere Exemplar hat eine abgeschatzte Lange von 
1.3 Meter. Die Schnauze ist lang, schmal, scharf zugespitzt, 
und die Augenhohlen scheinen verhaltnismassig gross zu sein. 



Introduction 

The majority of known ichthyosaur skeletons have been collected from the 
Lower Jurassic of Europe, principally from the Lower Lias (Hettangian to 
Sinemurian) of western England and the Upper Lias (Lower Toarcian) of 
southwestern Germany. Latipinnates predominate in the English Lower Lias, 



but the Upper Liassic ichthyosaurs appear to be exclusively longipinnates 
(McGowan, 1972a, 1972b). The present study is confined to the Lower 
Liassic latipinnates, all of which are referred to a single genus, Ichthyosaurus 
(see McGowan, 1974). 

The systematics of the Lower Liassic latipinnates is confused because de- 
scriptions are generally inadequate and often based upon non-diagnostic 
material such as isolated teeth and vertebral centra. Allometric growth is 
seldom taken into account, although Owen referred to this phenomenon when 
discussing Ichthyosaurus breviceps: "The specific characters are more fully 
exemplified in specimens of the skull of larger individuals, which show that 
the proportions of the rostrum to the rest of the skull in the smaller skeleton 
may be due to nonage..." (1881, p. 109). Lydekker (1889, p. 13) treats 12 
latipinnate species, the majority erected upon inadequate material. As four 
of these 12 species are from the Lower Lias of England: /. breviceps Owen, 
1881, /. conybeari Lydekker, 1888, /. communis Conybeare, 1821 and 
/. inter medius Conybeare, 1 822, they are of interest here. 

The original diagnoses for /. breviceps and /. conybeari, although some- 
what inadequate, were based upon reliable characters. The original diagnoses 
for /. communis and /. intermedius, however, were based upon tooth mor- 
phology, and the relative lengths of the angular and surangular, characters 
I have found to be unreliable. The teeth of Lower Jurassic ichthyosaurs are 
straight or gently curved cones with longitudinal striations on the base and 
crown. Some teeth are stout, others, especially towards the tip of the snout, 
are slender. The longitudinal striations range from faint lines to prominent 
ridges. It is often possible to observe the whole range of dental variations 
within the same dentition. Almost all dentitions have at least some teeth 
corresponding to Conybeare's figure for /. communis (1822, pi. 15, fig. 8). 
Further, of several thousand Liassic teeth I have now examined, less than ten 
correspond with Conybeare's figure for /. intermedius (1822, pi. 15, fig. 9) 
and these belong to only two specimens (R44 in the British Museum, J35 1 83, 
in the Oxford University Museum). The relative extent of the angular and 
surangular could be determined in less than half of the specimens examined 
and in only two of these did the angular extend further forward than the 
surangular (43971 and R1157, both in the British Museum). The present 
study is an attempt to overcome these inadequacies and bring order and 
stability to the systematics of the Liassic latipinnates. 



Materials and Methods 

Data were obtained from 66 latipinnate or presumed latipinnate ichthyo- 
saurs, all from the Lower Lias (Hettangian and Sinemurian) of England. 
The abbreviations used for the collections examined are: bmnh, British 
Museum (Natural History) ; IGS, Institute of Geological Sciences, London; 
OUM, Oxford University Museum; sec, Street Collection in the care of 
Clarks Shoe Company, Street, Somerset; scm, Somerset County Museum, 
Taunton, Somerset; and smc, Sedgwick Museum, Cambridge University. 

The state of preservation of these specimens was generally good, more 
than half being complete or near complete skeletons. All measurements 
were made with vernier calipers accurate to 0.01 cm, except those of body 
length which were taken with a tape accurate to 0.1 cm. Details of characters 
are given elsewhere (McGowan, 1974), and those which have been found 
to be of systematic importance are: length of jaw, length of snout, length of 
premaxillary segment, length of prenarial segment, diameter of orbit, internal 
diameter of sclerotic ring, length of external naris, number of maxillary 
teeth, length of forefin, width of forefin, number of primary digits (see 



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Fig. 1 Size ranges of Ichthyosaurus communis, I. conybeari, I. breviceps and /. 
tenuirostris ba'sed on jaw lengths. 
Solid box - observed range, bar - 95 per cent confidence limits. 



McGowan, 1972a), total digital count, maximum number of elements in 
longest digit, occurrence of notching in forefin elements, vertebral count from 
skull to tail bend, and vertebral count from skull to pelvis. 

Because of size variation in the sampled specimens it was necessary to 
convert the continuous variates into ratios. The cranial ratios used are: 
diameter of orbit to length of jaw, length of snout to length of jaw, length 
of premaxillary segment to length of jaw, length of prenarial segment to 
length of jaw, length of external naris to diameter of orbit, and internal 
diameter of sclerotic ring to diameter of orbit. The forefin ratio used is the 
aspect ratio, length of forefin to width of forefin. 

Using the above-mentioned characters, four distinct groups were estab- 
lished. Before testing their statistical significance, however, it was necessary 
to determine (a) whether the selected variates were normally distributed, 
and (b) whether they were affected by allometric growth. Because of small 
sample sizes, analyses could be conducted on only two of the four groups. 
The statistics gi and g2, for measuring skewness and kurtosis respectively 
were generated for each character within each group (see Sokal & Rohlf, 
1969) using the computer programme univar. No characters departed 
significantly from a normal distribution. An analysis of allometric growth, 
given elsewhere (McGowan, 1973) shows growth to be largely isometric. 
Hence the ratios used here are largely unaffected by the size of the individual. 

A Students t-test was used to make pairwise tests of the differences among 
group means for each character (see Simpson et al, 1960, p. 176) using 
the computer programme ttest. Probabilities are designated accordingly: 
P < 0.001 as ***, 0.01 > P > 0.001 as **, 0.05 > P > 0.01 as *. Pro- 
grammes UNIVAR and ttest available at rom. 

Results 

Four latipinnate species were recognized in the English Lower Lias, Ichthyo- 
saurus communis Conybeare, /. breviceps Owen, /. conybeari Lydekker, and 
/. tenuirostris Conybeare. Results of the statistical analyses are shown in 
Tables 1 and 2, and their size ranges are shown in Fig. 1. 



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Systematic Descriptions 

Class Reptilia 

Subclass Eiiryapsida(?) 

Order Ichthyosauria 

Suborder Latipinnati Von Huene, 1948 



Family Ichthyosauridae Bonaparte 1841 

Emended Diagnosis 

Forefin with no fewer than four primary digits, two of which originate from 
intermedium; total digital count usually not less than five; orbit relatively 
large, ratio, diameter of orbit to length of jaw > 0.20; aperture of the sclerotic 
ring relatively large, ratio, internal diameter of sclerotic ring to diameter of 
orbit usually > 0.35; maxilla relatively small, ratio, length of premaxillary 
segment to length of jaw usually > 0.40; pelvic girdle tripartite, rib articula- 
tions predominantly bicipital; Liassic in age and largely, if not wholly, con- 
fined to the Lower division (Hettangian and Sinemurian). 

Type Genus 

Ichthyosaurus De La Beche & Conybeare, 1821. 
Ichthyosaurus Koenig, 1818 nomen nudum. 



Ichthyosaurus De La Beche & Conybeare, 1821 

Emended Diagnosis 

Same as that for family. 

Type Species 

Ichthyosaurus communis Conybeare 



Ichthyosaurus communis Conybeare 
Figs. 2-3 

Ichthyosaurus communis De La Beche & Conybeare, 1821, p. 594. 
Ichthyosaurus communis ConyhtSiTQ, Conybeare, 1822, p. 108, pi. 15, fig. 8; 

pi. 16, fig. 8. 
Ichthyosaurus intermedius ConyhQa.Te, 1822, p. 108, pi. 17. 
Ichthyosaurus chiroparamekostinus Hawkins, 1834, pi. 17, pi. 22. 
Ichthyosaurus chiroparamekostinus Hav^k'ms, Hawkins, 1840, pi. 17, pi. 20, 

pi. 21. 
Ichthyosaurus latimanus Owen, 1840, p. 123. 
Ichthyosaurus communis Conybeare, Owen (pro-parte) , 1881, p. Ill, pi. 

30, fig. 3. 
Ichthyosaurus intermedius ConyhcsiTQ, Owen, 1881, p. 113, pi. 30, fig. 1. 
Ichthyosaurus communis Conybeare, Lydekker, 1889, p. 41. 
Ichthyosaurus intermedius Conybeare, Lydekker, 1889, p. 55. 







Fig. 2 Ichthyosaurus communis Conybesire, scale = 10 cm. 

A. BMNH R1162, skull of proposed neotype, Lower Lias, Lyme Regis. 

B. BMNH 39492, entire. Lower Lias, Lyme Regis, 
c. BMNH R1073, entire. Lower Lias, Lyme Regis. 

D. BMNH 2013, entire,iower Lias, Street (photograph laterally inverted), 







Fig. 3 Ichthyosaurus communis Conybeare, scale - 5 cm. 

Comparison of forefins of geologically older specimens, from Street (a, b) 
with those of geologically younger specimens from Lyme Regis (c, d) 

A. sec 15, Lower Lias, Street. 

B. BMNH 2013, Lower Lias, Street. 

c. BMNH R1073, Lower Lias, Lyme Regis. 

D. BMNH R1162 (proposed neotype). Lower Lias, Lyme Regis. 



8 



Eurypterygius communis (Conybeare), VonHuene (pro-parte) , 1922, p. 5. 
Eurypterygius intermedins (Conybeare), Von Huene, 1922, p. 9, pi. 1, 
fig. 5. 

Emended Diagnosis 

Vertebral count to tail bend 77 ± 2; vertebral count to pelvis 44 ±: 3 but 
usually < 46; ratio, diameter of orbit to length of jaw 0.20-0.28 but usually 
< 0.25; ratio, length of snout to length of jaw 0.57-0.69; ratio, length of 
premaxillary segment to length of jaw 0.38-0.48 but usually exceeding 0.40; 
ratio, length of prenarial segment to length of jaw 0.40-0.56 but usually 
exceeding 0.43; internal diameter of sclerotic ring to diameter of orbit usually 
> 0.35; maxillary teeth 14 ± 4; primary digits ^ 4; elements in longest digit 
^17; notching may occur in fore- or hindfin elements. 

Original Type Material 

A partial skeleton from Lyme Regis, Dorset, briefly described by Conybeare 
1821, p. 594, now lost (see Remarks). 

Proposed Neotype 

BMNH R1162, an almost complete skeleton in the British Museum (Natural 
History), London. 

Consistency of Neotype with Original Type-Material 

The original type-material was found at Lyme Regis, and since only the 
lower division of the Lias outcrops there, it is certain that the specimen was 
Lower Liassic in age (the tooth figured by Conybeare in 1822 was also from 
the Lias, probably from Lyme Regis). It is reasonably certain that the 
specimen was not a longipinnate because all the known longipinnates are 
much larger than Conybeare's (1821) specimen. Conversely, Conybeare's 
specimen is too large to be referred to /. breviceps or /. conybeari. No other 
latipinnates occur at Lyme Regis. The proposed neotype is from Lyme Regis, 
and the jaw size of Conybeare's (1821) specimen falls within the size 
range of the taxon which includes the neotype. The proposed neotype is 
therefore consistent with what is known of the original type-material. (It is 
also consistent with Conybeare's figure of the tooth of /. communis, 1822, pi. 
15, fig. 8). 

Other Material 

From the 39 specimens which I refer to this species may be cited those which 
have contributed most to the emended description: 

A. From Lyme Regis, Dorset: bmnh R1073, bmnh 36256, bmnh R12, 
BMNH R1071, BMNH 39492. The first three are almost complete skeletons, 
the fourth has some post-cranial structures including a forefin, and the fifth 
is a particularly well-preserved skull. 

B. From Street, Somerset: bmnh 2013, bmnh 2013a, bmnh R44, bmnh 
14565, SMC J35183, smc J59575. All are complete or nearly complete 
skeletons, bmnh 2013 is particularly well preserved and probably the finest 
latipinnate specimen from the Lower Lias. 



Locality and Horizon 

/. communis is known from Lyme Regis in Dorset, Street in Somerset, and 
Barrow-on-Soar in Leicestershire. As noted elsewhere (McGowan, 1974) 
the geological range of ichthyosaurs collected from Lyme Regis may be 
taken to be from the ammonite zones of Schlotheimia angulata to Arnioceras 
semicostatum (Upper Hettangian - Lower Sinemurian), Fig. 4. The Somer- 
set locality is slightly older (geologically) than the Dorset locality. The 
horizon exposed at the old quarry of Street was of the ammonite zone of 
Psiloceras planorbis, and Arkell (1933) noted that most of the reptiles were 
collected from the Fre-planorbis beds which are at the base of this zone 
(Lower Hettangian). There is, therefore, a small temporal separation be- 
tween the two localities, represented by the zone of Psiloceras planorbis. 

Emended Description 

/. communis, the commonest Liassic latipinnate was a medium-sized species, 
reaching a total length of about 2.5 m. The small differences between the 
Dorset specimens and those from the geologically older Somerset locality 
are in the forefin and are consistent with the evolutionary trends discussed 
elsewhere (McGowan, 1972b). 

SKULL 

Snout, which constitutes over half the length of skull, tapers gently towards 
tip. Mean value for ratio, length of snout to length of jaw 0.63, 0.59 in 
neotype. Maxilla short, wedge-shaped, bearing no more than 18 teeth. Ratio, 
length of maxillary segment to length of jaw 0.41 in neotype, mean 0.43. 
External narial aperture (frequently indistinct) bounded by lachrymal, pre- 
maxilla, and nasal. Orbit fairly large, ratio, diameter of orbit to length of 
jaw > 0.20, 0.25 in neotype, mean 0.23. 



STAGES 


ZONES 


LITHOLOGICAL 
FORMATIONS 


LOCALITIES 


UPPER 
URIAN 


Echioceras raricostatum 


Black Marl of 
Black Ven 


Lyme Regis 
Dorset 


Oxynoticeras oxynofum 


Asteroceras obtusum 


LOWER 
SINEM 


Caenisites turnerl 


Shales-with-Beef" 


Arnioceras semicostatum 


Blue Lias 


Arietites bucklandi 


z 
< 

o 

z 

? 

1- 

LU 

X 


Schlotheimia angulata 


Alsatites liasicus 






Psiloceras planorobis 




Street 
Somerset 



Fig. 4 The ammonite zones of the English Lower Lias (from McGowan, 1974). 
10 



FOREFIN 

In Somerset specimens forefin is rectangular rather than rounded (Fig. 3 A, 
B); individual elements tend to be pentagonal or hexagonal rather than 
discoidal and are fewer in number, with not more than 20 elements in longest 
digit. Notching usually occurs in some elements of first digit, no notching 
in Dorset material. Number of primary digits not exceeding five in Somerset 
specimens, but usually six or seven in Dorset material. In neotype (bmnh 
R1162), from Dorset, forefin has rounded outline, possesses six primary 
digits, with 26 elements in the longest digit (Fig. 3D). Forefin structure quite 
variable within species with wide range of variation, both in number of digits 
and elements, and in their spatial configuration. This plasticity is exemplified 
in a skeleton from Somerset (oum J 13799) where left fin is quite distinct 
from right (Fig. 5). 

AXIAL SKELETON 

Vertebral count to tail bend 77 in the neotype, mean value 77, observed 
range 74-79. Total vertebral count indeterminate in the neotype, but mean 
value approximately 135. Height of neural arch and spine relative to centrum 
somewhat less than in /. breviceps, but this is not a reliable character because 
vertebral proportions vary according to position in column. Neural arch and 
spine become relatively taller with increasing distance from skull, reach a 
maximum in posterior thoracic region, then gradually decrease in height 
until they disappear at some point distal to tail bend. 




Fig. 5 Ichthyosaurus communis Conybcare, scale = 10 cm. 

Comparison of right (a) and left (b) forefins of an individual (gum J13799) 
showing an extreme case of structural variability. Ventral view (for ease of 
comparison left fin laterally inverted). 

11 



LIMB GIRDLES 

Structure of pectoral and pelvic girdles known for a number of specimens. 
Pectoral girdle formed from four elements, paired coracoids, scapulae, and 
clavicles, and a single median interclavicle. Coracoid has anterior and pos- 
terior notches (Fig. 6A). Pelvic girdle tripartite with separate ilium, ischium, 
and pubis (Fig. 6B). Ilium and ischium are slender rods of bone, whereas 
pubis has much expanded distal and proximal heads. As girdles are frequently 
incomplete they are of reduced systematic interest. 

RIBS 

Heads of the ribs not visible in neotype, but some, exposed in bmnh 36256, 
are bicipital. 

DENTITION 

Teeth generally fairly stout, gently recurved, with fine striations on crown 
and coarse striations on base. Anterior teeth more slender than those near 
angle of jaw. Because tooth characters are variable they are of little systematic 
importance. 

Remarks 

The first description of /. communis is usually attributed to Conybeare's 
1822 paper (Woodward and Sherborn, 1890, p. 236; Kuhn, 1934, p. 32), 
but the first material assigned to the species was that of "the head, with all, 
or nearly all, the vertebrae in place,'' from Lyme Regis (Conybeare, 1821, p. 





Fig. 6 Ichthyosaurus communis Conybeare, limb girdles. 

A. Pectoral girdle, based upon gum J 13799, scale = 10 cm: cl, clavicle; cor, 
coracoid; icl, interclavicle; sc, scapula; an, anterior notch; pn, posterior 
notch; gl, glenoid cavity. 

B. Pelvic girdle, based upon bmnh R3372, scale = 5 cm: il, ilium; is, ischium; 
pb, pubis. 



12 



594). No figure was given, but sufficient information was provided to enable 
the specimen to be recognized. No trace of this specimen can be found, and 
although the description is inadequate, it is no less diagnostic than Cony- 
beare's 1822 description. I therefore conclude that the name first became 
available in 1821. As the description was inadequate (so too was the 1822 
description), the name /. communis is a nomen dubium. It is thus to be 
rejected, and one of the synonyms adopted, or a case made for its retention. 
As the name /. communis has been in use for a considerable time, and as the 
original descriptions of the synonyms were inadequate, there is a good case 
for retaining /. communis. Retention was accomplished above by proposing 
a neotype consistent with what is known of the original type material. 

Ichthyosaurus latimanus Owen 1840 has usually been synonymized with 
/. conybeari (Woodward and Sherborn, 1890, p. 237; Lydekker, 1889, p. 53; 
Kuhn, 1934, p. 34), but this is incorrect. The holotype of /. latimanus was a 
"nearly complete but dislocated skeleton", and Owen gave a number of 
measurements which he compared with those taken from a specimen of 
/. communis (Owen 1840, p. 123). /. latimanus was said to differ from 
/. communis in having a relatively larger forefin and smaller skull. However, 
if the ratio, length of entire forefin (i.e. including humerus) to the length of 
the entire body (i.e. including skull), is calculated from Owen's data, the 
values obtained for the holotype of /. latimanus and for Owen's specimen of 
/. communis are similar (0.09 and 0.08 respectively). Furthermore the 
relative length of the forefin of /. latimanus falls within the observed range 
for the species /. communis. The skull of Owen's holotype of /. latimanus was 
relatively smaller than in the specimen of /. communis with which he com- 
pared it, but this discrepancy can be explained in terms of differential growth. 
The holotype was the larger of the two skeletons, and the head of /. communis 
grows with a negative allometry (McGowan, 1973). As there are no in- 
consistencies between Owen's type material and /. communis, it is concluded 
that /. latimanus is a synonym for /. communis. The type specimen of /. 
latimanus was destroyed when the Bristol Museum was bombed during the 
last war, but according to Lydekker (1889, p. 53) bmnh 1065 is a cast of 
this specimen. It may be noted that there are inexplicable discrepancies 
between the original (collected from the Lower Lias of Saltford, Somerset) 
and the cast. According to Owen (1840, p. 123) the original specimen pos- 
sessed a forefin, but Lydekker observed (on bmnh 1065) that "the only 
traces of the pectoral Hmb are a few phalangeals". Both forefins are present 
in the cast today, but they do not correspond with the original measurements 
given by Owen, although there is good agreement elsewhere. 

Author's measurements Owen's measurements (1840) 
on BMNH 1065 for holotype of /. latimanus 

(cm) (cm) 

Length of jaw 42 40 

Length of body (minus skull) 164 165 

Width of forefin 5 9 

As BMNH 1065 compares closely with Owen's description of the original 
specimen it probably is indeed a cast of the original, but the forefin discrep- 
ancy remains unexplained. It seems that for some unknown reason the fore- 

13 



fins were not duplicated in the original cast, but a pair of forefins have been 
added since Lydekker's time, either to the original cast, or perhaps to a 
new one. 

Two populations are recognized within /. communis, one from Lyme 
Regis, Dorset, the other from the geologically older locality of Street, Somer- 
set. Individuals of the geologically older population (from Somerset) tend 
to be larger than those from Dorset, and, as the allometric growth constant 
for the head relative to the body is less than one (McGowan, 1973), they 
have relatively smaller heads. These larger individuals have previously been 
referred to the species /. intermedius, and Von Huene (1922, p. 9) regarded 
the possession of a relatively smaller head as a diagnostic character of the 
species. Other differences between the two populations include minor varia- 
tions in the skull and forefin, and in both instances the geologically older 
individuals are the more primitive (for a discussion of evolutionary trends 
in ichthyosaurs see McGowan, 1972b) . Thus individuals from Somerset tend 
to have relatively smaller orbits and larger maxillae, but these differences 
are not statistically significant. More striking is the variation in forefin 
structure : in the older population the number of elements in the longest digit 
does not exceed 20, there are no more than five primary digits, and notching 
almost always occurs in at least one or two elements. Notching never occurs 
in the Dorset material, the number of primary digits exceeds five, and the 
number of elements in the longest digit exceeds 20. 

The temporal separation between these two localities is represented by 
the ammonite zone of Psiloceras planorbis, which reaches a maximum thick- 
ness of 30 metres in Somerset (Arkell, 1933, p. 122). While any estimate 
of sedimentation rate in the Lower Lias would be speculative, some indica- 
tion of the temporal separation represented by these 30 metres of shales and 
limestones can be obtained by reference to other neritic marine deposits for 
which radiometric data are available. The rates of deposition in the shallow 
epeiric sea which covered much of North America during the Cretaceous 
vary from 344 feet (105 m) per million years for the Pierre Shale in 
Wyoming (Gill and Cobban, 1966) through 256 feet (78 m) per million for 
the Bearpaw Formation of Southern Saskatchewan (Caldwell, 1968) to 150 
feet (46 m) per million years in Alberta for the interval Middle Albian to 
Upper Maestrichtian (Folinsbee et ai, 1961). If similar deposition rates 
could be assumed for the English Liassic deposits, which were similarly laid 
down in a shallow (though not epicontinental) sea, the temporal separation 
between the Dorset and Somerset localities would lie somewhere between 
0.3 and 0.7 million years. 

As the differences between the two populations are small and as the tem- 
poral separation probably did not amount to more than half a million years, 
it seems prudent to treat them as part of an evolutionary continuum not 
worthy of taxonomic distinction. 



Ichthyosaurus breviceps Owen 
Figs. 7-8 

Ichthyosaurus breviceps Owen, 1881, p. 109, pi. 29, fig. 2. 
Ichthyosaurus breviceps Owen, Lydekker, 1888, p. 311. 

14 



o 






Fig. 7 Ichthyosaurus breviceps Owen, scale = 20 cm. 

BMNH 43006, holotype. Lower Lias, Lyme Regis. 

A. Skull. 

B. Entire specimen. 

c. Forefin, left dorsal view. 



15 






Fig. 8 Ichthyosaurus breviceps Owen, scale = 6 cm. 

A. BMNH 39263, entire (photograph laterally inverted). Lower Lias, Lyme 
Regis. 

B. BMNH R3367, skull, Lower Lias, Lyme Regis, 
c. BMNH R216, skull, Lower Lias, Lyme Regis. 



16 



Ichthyosaurus breviceps Owen, Mansell-Pleydell, 1888, p. 19. 

Ichthyosaurus breviceps Owen, Lydekker, 1889, p. 52. 

Eurypterygius breviceps (Owen), Von Huene, 1922, p. 8, pi. 1, fig. 1, fig. 2. 

[non] Ichthyosaurus breviceps Owen, 1881, pi. 29, fig. 1. 

[non] Eurypterygius breviceps (Owen), Von Huene, 1922, pi. 1, fig. 3. 

Emended Diagnosis 

Vertebral count to tail bend 68 ± 2; vertebral count to pelvis < 46; ratio, 
diameter of orbit to length of jaw > 0.25; ratio, length of snout to length of 
jaw < 0.57; ratio, length of premaxillary segment to length of jaw 0.33 — 
0.44, but frequentiy < 0.40; ratio, length of prenarial segment to length of 
jaw usually < 0.43; ratio internal diameter of sclerotic ring to diameter of 
orbit > 0.35; maxillary teeth probably ^10; primary digits ^ 5; elements 
in longest digit probably > 25 ; notching probably does not occur in fore- or 
hindfin elements. 

Holotype 

BMNH 43006, a complete skeleton in the British Museum (Natural History), 
London, described and figured by Owen (1881, p. 109, pi. 29). 

Other Material 

The following specimens have contributed to the emended description: 
BMNH R216, BMNH 8437, BMNH R1074, BMNH R3367, BMNH 39263 and 
BMNH 33277. The first three specimens are complete or nearly complete 
skeletons, the fourth is a skull and partial pectoral girdle with some vertebrae 
and ribs, the fifth a particularly well-preserved skull, and the sixth is a partial 
skull with incomplete post-cranial skeleton. 

Locality and Horizon 

With the exception of bmnh 8437, for which no information is available, all 
material examined was collected from Lyme Regis, Dorset. Owen (1881) 
states that the species has been found in the neighbourhood of Brownish, 
Somerset, from the zone of Arietites bucklandi. I. breviceps therefore occurs 
in the upper portion of the Blue Lias, from the zone of Schlotheimia angulata 
to that of Arnioceras semicostatum (Upper Hettangian - Lower Sinemurian). 

Emended Description 

/. breviceps was a small species reaching a maximum length of about 1.5 m. 
It was uncommon, and, to date, only seven specimens have been positively 
identified. 

SKULL 

Reduced snout and large orbit clearly differentiates this species from /. com- 
munis and all other latipinnates. Ratios, diameter of orbit to length of jaw, 
and length of snout to length of jaw 0.31 and 0.55 respectively in holotype 
(mean values 0.30, 0.53). Brevity of snout accentuates downward curvature 
giving strikingly avian appearance (Figs. 7, 8). Ratio, length of premaxillary 
segment to length of jaw 0.44 in holotype, mean 0.39. External naris tends 
to be relatively larger than in /. communis: ratio, length of external naris to 
diameter of orbit 0.36 in holotype, mean value 0.38. 

17 



FOREFIN 

Structure of forefin known in holotype, in bmnh 8437, and in bmnh R216. 
Primary digits number between five (holotype) and six (bmnh R216), total 
digital count between eight and nine. Number of elements in longest digit 27 
in both holotype and bmnh 8437. No notching in fore- or hindfin elements. 

AXIAL SKELETON 

Vertebral count to tail bend, indeterminate in holotype, is 69 or 70 in bmnh 
8437, and estimates for bmnh R216 and bmnh 1074 are 67, and 65 to 67, 
respectively. Total vertebral count approximately 116 for holotype, 122 for 
bmnh R216 (mean value 135 in /. communis) . 

LIMB GIRDLES 

Because of poor preservation or exposure, structure of girdles is not well 
known. In holotype pectoral girdle is represented by two poorly preserved 
scapulae and part of a clavicle. Pelvic girdle indeterminate. 

RIBS 

Rib heads obscured in holotype, but those exposed in bmnh 33277 and 
BMNH R1074 are bicipital. In bmnh R216 rib heads are again obscured, but 
double rib facets can be seen on a number of centra. 

DENTITION 

Teeth of holotype have smooth bases, with fine striations on crown. Shape 
similar to those of /. communis. 



Remarks 

Owen included bmnh R1 157 in his original description of /. breviceps, but 
this specimen is a longipinnate and is described elsewhere as a new species 
(McGowan, 1974). bmnh 2001 might belong to the present species, but 
the skull is so broken and distorted that no measurements can be made and 
the specimen must remain unidentified. This specimen is of historic interest 
because it was figured by Hawkins (1834, pi. 6) as the type of Ichthyosaurus 
chiropolyostinus. If this specimen were referred to /. breviceps it would 
extend the upper limit of the size range by about 20 per cent. 



Ichthyosaurus conybeari Lydekker 
Figs. 9-10 

Ichthyosaurus conybeari Lydekker, 1888, p. 312. 

Ichthyosaurus conybeari hydQkkQT, Lydekker, 1889, p. 53, fig. 22. 

Eurypterygius conybeari (Lydekker), Von Huene, 1922, p. 8, pi. 13, fig. 9. 

Emended Diagnosis 

Vertebral count to tail bend 79; vertebral count to pelvis < 46; ratio, dia- 
meter of orbit to length of jaw probably < 0.25; ratio, length of snout to 
length of jaw > 0.57; ratio, length of premaxillary segment to length of jaw 

18 




Fig. 9 Ichthyosaurus conybeari Lydekker, scale = 20 cm. 
BMNH 38523, holotype, Lower Lias, Lyme Regis. 



19 




Fig. 10 Ichthyosaurus conybeari Lydekker, scale = 20 cm. 
IGS 956, Lower Lias, Lyme Regis. 

A. Entire specimen. 

B. Skull. 

c. Forefin, left dorsal view. 

Crown Copyright Geological Survey photograph. 

Reproduced by permission of the Controller, Her Majesty's Stationery Office. 



20 



probably > 0.40; ratio, length of prenarial segment to length of jaw proba- 
bly > 0.43; ratio, internal diameter of sclerotic ring to diameter of orbit 
> 0.35; maxillary teeth > 18; primary digits probably 5; elements in longest 
digit < 25; notching occurs in forefin elements. 

Holotype 

BMNH 38523, a poorly preserved and incomplete skeleton, comprising skull, 
forefin, pectoral girdle, and partial axial skeleton, in the British Museum 
(Natural History), London. 

Other Material 

IGS 956, a complete and well-preserved skeleton in the Institute of Geological 
Sciences, London. 

Locality and Horizon 

Both specimens were collected from Lyme Regis, Dorsetshire, and the geo- 
logical range of the species is taken to be from the zone of Schlotheimia 
angulata to Arnioceras semicostatum (Upper Hettangian - Lower Sine- 
murian: Fig. 4). 

Emended Description 

/. conybeari, with but two known representatives, is the rarest Liassic latipin- 
nate. Based upon a single complete specimen (igs 956), with total length of 
only 87 cm, it appears to be a small species, but this might be an immature 
individual. The larger specimen (bmnh 38523) has jaw length comparable 
with /. communis, but exceeds range for /. breviceps (see Fig. 1). /. cony- 
beari differs significantly from /. breviceps (Table 2) in having a relatively 
smaller orbit, longer snout, and prenarial and premaxillary regions, and in 
possessing notched forefin elements. Only significant differences from /. com- 
munis are lower vertebral count to pelvis and higher maxillary tooth count. 
Forefin relatively smaller compared with body length than in /. communis 
but not statistically significant. However, in contrast to specimens of /. com- 
munis from same locality (Lyme Regis) as /. conybeari, forefin has notched 
elements and number of elements in the longest digit does not exceed 20. 
For these reasons /. conybeari is considered to be a distinct species from 
/. breviceps and /. communis. 

I. conybeari, with its long and slender snout bears a superficial resem- 
blance to /. tenuirostris, and its smaller size might suggest it was merely a 
juvenile form of /. tenuirostris. There are, however, mutually exclusive values 
in certain discontinuous variates. The vertebral count to the pelvis, 40 in 
/. conybeari, is significantly lower than in /. communis (Table 2), which in 
turn is significantly lower than in /. tenuirostris. Furthermore the primary 
and total digital counts, and the maximum number of elements in the longest 
digit are all higher in /. conybeari than in /. tenuirostris (Table 1 ) . Small 
sample sizes prevent a statistical treatment, but it seems that there were 
probably significant differences in some continuous variates. Thus the ratios, 
length of snout to length of jaw, length of premaxillary segment to length of 
jaw, and length of prenarial segment to length of jaw are all much lower in 

21 



/. conybeari than in /. tenuirostris. There can therefore be little doubt that 
they are distinct species. 

SKULL 

Because of poor preservation of holotype most information has been obtained 
from IGS 956. Holotype skull appears very long compared with rest of body, 
and snout is long and slender but cannot be measured accurately. In IGS 956 
ratio, length of snout to length of jaw is 0.65, which exceeds most specimens 
of /. communis. Snout and lower jaw, both very slender, taper to point. Orbit 
appears relatively large but ratio, diameter of orbit to length of jaw only 
0.23. Aperture of sclerotic ring, probably distorted, is elliptical, and ratio, 
internal diameter of sclerotic ring to diameter of orbit is 0.40, comparable 
with /. communis and /. breviceps. Maxilla, slender and splint-like, tapers 
to sharp point anteriorly and bears 21 teeth. Ratio, length of premaxillary 
segment to length of jaw 0.47, which exceeds most specimens of /. communis. 
External narial aperture set well back: ratio, length of prenarial segment to 
length of jaw 0.53, exceeding that of most specimens of /. communis. 

FOREFIN 

In both holotype and IGS 956 forefin has five primary digits, one accessory, 
with 19 elements in longest digit. Notching occurs in both specimens in at 
least two elements of first digit. Forefin relatively small compared with 
body length. 

HINDFIN 

Hindfin, well preserved in los 956, has five primary digits and one post-axial 
accessory digit. Anterior margin of tibia emarginated, no indication of notch- 
ing elsewhere. 

AXIAL SKELETON 

Vertebral count to tail bend 79 in IGS 956. (Indeterminate in holotype.) 

LIMB GIRDLES 

Type specimen with a pair of well-preserved coracoids and a scapula, but 
pectoral girdle largely indeterminate in IGS 956. Width of coracoid less than 
anterior-posterior length. Anterior and posterior notches present, former 
approximately T width of coracoid. Scapula of moderate size, maximum 
width almost half length. Clavicle gently curved and fairly stout. Only one 
small slender pelvic element (possibly the ilium) well exposed, somewhat less 
than length of femur. 

RIBS 

Exposed ribheads (in holotype) bicipital. 

DENTITION 

Teeth fairly stout recurved cones with smooth bases and finely striated 
crowns, much like those of /. breviceps. Teeth much more slender towards 
tip of snout than at angle of jaw. 

22 








Fig. 1 1 Ichthyosaurus tenuirostris Conybeare, scale =25 cm. 
IGS 51236, proposed neotype, Lower Lias, Street. 

A. Entire specimen. 

B. Skull and pectoral girdle. 

Crown Copyright Geological Survey photograph. 

Reproduced by permission of the Controller, Her Majesty's Stationery Office. 

23 



i_ 



A 





Fig. 12 Ichthyosaurus tenuirostris Conybeare, scale = 15 cm. 

A. BMNH R498, Lower Lias, Street. 

B. ouM J 10305, Lower Lias, Street. 

24 



Ichthyosaurus tenuirostris Conybeare 
Figs. 11-12 

Ichthyosaurus tenuirostris ConyhQare, 1822, p. 108, pi. 15, fig. 10. 
Ichthyosaurus tenuirostris Conybeare, De la Beche, 1826, p. 27. 
Ichthyosaurus chirostrongulostinus Hawkins, 1834, pi. 13. 
Ichthyosaurus chirostrongulostinus Hawkins, 1840, pi. 13. 
Ichthyosaurus tenuirostris ConyhQarQ, OwQn, 1840, p. 117. 
Ichthyosaurus tenuirostris ConybQSLTQ, Owen, 1881, p. 122, pi. 24, fig. 8; 

pi. 25, fig. 3; pi. 32, figs. 1-6. 
Ichthyosaurus tenuirostris ConybQarQ, Mansell-Pleydell, 1888, p. 19. 
Ichthyosaurus tenuirostris Conybeare, Lydekker, 1888, p. 311. 
Ichthyosaurus tenuirostris Conybeare, Lydekker, 1889, p. 83, fig. 25. 
Leptopterygius tenuirostris (ConyhearQ), Von Huene, 1922, p. 11, pi. 2, 

fig. 1, fig. 3. 

Emended Diagnosis 

Vertebral count to tail bend > 79; vertebral count to pelvis ^ 46; ratio, 
diameter of orbit to length of jaw < 0.25; ratio, length of snout to length of 
jaw > 0.70; ratio, length of premaxillary segment to length of jaw > 0.48; 
ratio, length of prenarial segment to length of jaw > 0.56; ratio, internal 
diameter of sclerotic ring to diameter of orbit > 0.35; teeth predominantly 
slender; 4 primary digits in forefin; elements in longest digit probably < 17; 
radius notched; humerus with constricted shaft and expanded distal head; 
occlusal edges of radius and ulna may enclose small foramen; individual 
forefin elements relatively large, few in number, well spaced towards apex 
of fin; femur with slender shaft and wide distal head; tibia and tibiale notched. 

Original Type-Material 

A tooth figured by Conybeare (1822, pi. 15, fig. 10) which was in the collec- 
tion of the Geological Society (Woodward and Sherborn, 1890, p. 240) but 
which now appears lost (Delair, 1960, p. 70). 

Proposed Neotype 

IGS 51236, a complete skeleton in the Institute of Geological Sciences, Lon- 
don (Fig. 11). 

Consistency of Neotype with Original Type-Material 

The long slender snout is consistent with Conybeare's description (1822, p. 
108), and many of the teeth correspond in general shape to original type 
specimen. 

Other Material 

BMNH R498, an entire and well-preserved specimen, OUM J 10305, a partial 
skeleton, and SCM 8372, a complete skeleton. 

Locality and Horizon 

The original description did not give the type locality. The proposed neotype, 
OUM J 10305, BMNH R498, and scm 8372 were all collected from Street in 

25 



Somerset and are therefore probably from the Frt-planorbis beds. Lydekker 
(1889, pp. 83-88) listed a number of specimens of /. tenuirostris from Lyme 
Regis, but these are largely fragmentary and cannot definitely be assigned to 
this taxon. The same is true of specimen bmnh 36876 which was collected 
from the Upper Lias of Whitby. Specimen bmnh R1120, figured by Owen 
(1881, pi. 32, fig. 1) is probably a member of the present species and is 
referred to by Lydekker (1889, p. 83) as being, "probably from the Lower 
Lias of Lyme Regis". It is therefore concluded that this species is probably 
from the lowermost beds of the Lower Lias, but the possibility that it ex- 
tended further up into the Lower Lias cannot be excluded. The range will 
therefore be taken as extending from the VxQ-plartorbis beds, possibly to the 
zone of Echioceras raricostatum (Lower Hettangian - Upper Sinemurian[?]). 

Description of Neotype and Other Material 

/. tenuirostris is not an uncommon ichthyosaur but as there are so few com- 
plete specimens it is not a very well-known species. It was probably some- 
what larger than /. communis, reaching a maximum length of about three 
metres. 

SKULL 

Snout and lower jaw extremely slender; ratio, length of snout to length of 
jaw 0.72 in neotype, 0.73 in oum J 10305, and approximately 0.76 in bmnh 
R498. Orbit occupies much of post-rostral portion of skull, but, as snout is 
very long, orbit small relative to length of skull; ratio, diameter of orbit to 
length of jaw approximately 0.23 in neotype, 0.22 in oum J 10305. Ratio, 
internal diameter of sclerotic ring to diameter of orbit 0.41 (oum J10305). 
Small size of maxilla reflected in large value for ratio, length of premaxillary 
segment to length of jaw 0.52 in neotype and in oum J 10305. Ratio, length of 
external naris to diameter of orbit 0.44 in oum J 10305, probably 0.37 in 
neotype (mean value 0.38 in both /. communis and /. breviceps). Ratio, 
length of prenarial segment to length of jaw 0.60 in neotype, 0.61 in oum 
J 10305. Forty-four teeth in upper jaw of oum J 10305, but uncertainty 
regarding possession of maxillary teeth; skull figured by Owen ( 1881, pi. 32, 
fig. 2) appears to have them. 

FOREFIN 

Four primary but no accessory digits, maximum number of elements in 
longest digit, 15 in bmnh R498, may not exceed 16. Individual elements 
rounded, relatively large, few in number, well spaced distally. Small foramen 
enclosed between radius and ulna in oum J 10305 and third and fourth distal 
carpals fused in left fin. Lydekker (1889, p. 83) noted that radius and ulna 
frequently ankylose. Distal head of humerus much expanded giving a con- 
stricted appearance to midshaft region. Forefins unreliable in neotype as 
they have been reconstructed distally. 

HINDFIN 

Hindfin with only three digits; tibia and tibiale emarginated. Femur slender 
with expanded distal head. 

26 



AXIAL SKELETON 

Vertebral count to tail bend 85 in oum J10305, 79 or 80 in SCM 8372. 
In neotype and bmnh R498 the character is largely indeterminate but there is 
evidence of constrictions at 84th and 85th vertebrae respectively, probably 
marking position of tail bend. Vertebral count to pelvis 47 in neotype, 49 or 
50inBMNHR498. 

LIMB GIRDLES 

Girdles largely obscured in bmnh R498 and oum J 10305, but well exposed 
in neotype where coracoids are rectilinear rather than rounded in outline 
and with single (anterior) notch. 

DENTITION 

Many teeth slender, others relatively stout recurved cones resembling those 
seen in other species. 



Remarks 

/. tenuirostris has hitherto been referred to the longipinnates (Lydekker 
1889, Von Huene 1922, Kuhn 1934), but it probably has greater affinity 
with the latipinnates. One of the major problems in resolving its true affinity 
has been the interpretation of the forefin. In bmnh R498 the forefin has an 
overall longipinnate appearance (McGowan 1972a) in that there are only 
15 elements in the longest digit, the radius is notched, and the individual ele- 
ments are relatively large and well spaced distally. It is difficult to determine 
the number of primary digits; in the left forefin the fourth digit appears to 
be an accessory digit, originating from the lateral margin of the third distal 
carpal, having litde or no contact with the ulnare. Conversely, in the right 
forefin, the fourth digit originates from a distinct facet on the ulnare and is, 
by definition, a primary digit. Thus there are three primary digits in the left 
forefin and four in the right. A similar situation is apparent in IGS 51236 
where the left forefin has four primary digits and the right, three primary and 
one post-axial accessory digits. In a third specimen, oum J 10305, the fins 
are incomplete but the left fin has four primary digits. These different digital 
counts can be reconciled if it is assumed that there are four primary digits and 
that the articulation of the fourth digit (with the ulnare) is not always well 
developed. 

Corroborative evidence for the latipinnate status of /. tenuirostris is avail- 
able from the skull. In oum J 10305 the ratio, diameter of orbit to length of 
jaw is 0.22, the ratio, internal diameter of sclerotic ring to diameter of orbit 
is 0.41, and the ratio, length of premaxillary segment to the length of jaw 
is 0.52 (also 0.52 in neotype) . All of these values lie well within the predicted 
range for Lower Liassic latipinnates (McGowan, 1972a, p. 7). It is there- 
fore concluded that /. tenuirostris is a latipinnate. 

The original description of /. tenuirostris, although somewhat inadequate, 
did include reference to the long snout, "... the species is best marked by the 
extreme length and thinness of the snout, in which points it very strikingly 
exceeds all the other ichthyosauri" (Conybeare, 1822, p. 108); and I con- 
sider this to be a valid description. 

27 



Conclusions 

The latipinnate ichthyosaurs of the EngUsh Lower Lias, which far outnumber 
their longipinnate contemporaries, are represented by four species: Ichthy- 
osaurus communis, I. breviceps, I. conybeari, and /. tenuirostris. I. com- 
munis, the most common species, accounts for about half of the ichthyosaurs 
found in England. Most specimens referred to /. communis were collected 
from Lyme Regis, Dorset (Upper Hettangian - Lower Sinemurian) and can 
be distinguished from the geologically older population collected from Street, 
Somerset (Lower Hettangian). /. communis was a medium-sized species 
reaching a maximum length of about 2.5 m though most specimens are less 
than 2 m./. communis is similar in size and general proportions to the Ger- 
man longipinnate Stenopterygius quadriscissus, the most common Upper 
Liassic species. Latipinnates are not found in the Upper Lias and perhaps 
S. quadriscissus was a faunal replacement for /. communis. 

I. breviceps is not a common species, and only seven specimens are known 
to date, all of which were collected from Lyme Regis. It is much smaller than 
/. communis, the maximum length being about 1.5 m. The species is charac- 
terized by the abbreviated snout and large orbit. 

/. conybeari is rare, with only two known specimens, both from Lyme 
Regis. Like /. breviceps this was probably a small species, the larger of the 
two individuals having an estimated body length of less than 1.5 m. The 
species is characterized by its long thin snout and relatively small forefins. 

Although /. tenuirostris is represented by only four complete skeletons, 
a number of isolated epipodials are known and it would seem that it was 
not an uncommon species. Slightly larger than /. communis, the maximum 
body length was about 3 m. The species is characterized by its remarkably 
long, thin snout. 

The Lower Liassic latipinnates were thus small to medium-sized ichthyo- 
saurs, and far outnumbered their larger longipinnate contemporaries 
(McGowan, 1974). It may not have been through some caprice of the fossil 
record that latipinnates outnumber longipinnates in the Lower Lias. Longi- 
pinnates may have occupied a different ecological niche, they may have been 
oceanic rather than neritic, solitary rather than gregarious; or their large 
size may have produced lower population densities. The success of the 
latipinnates during that early part of the Jurassic is measured in the great 
wealth of material which they left behind. It is therefore somewhat surprising 
that nothing more is seen of latipinnates until the Upper Jurassic where 
O phthalmosaurus occurs. Perhaps this gap is a reflection of a failure of the 
fossil record rather than of the latipinnates, and only further material can 
answer this question. 



A ckno wiedgments 

I wish to express my sincere thanks for the help and hospitality I received 
from the staffs of the many institutions visited. I particularly wish to thank 
Dr. Alan Charig, Mr. Cyril Walker, Miss Marilyn Holloway and Mr. Philip 
Palmer of the British Museum (Natural History); Dr. Colin Forbes of the 

28 



Sedgwick Museum and Mr. Peter Biddlestone formerly of the Sedgwick 
Museum, Cambridge, and of the Royal Ontario Museum; Dr. Ivimey-Cook, 
and Mr. Martin Pulsford of the Institute of Geological Sciences, London; 
Mr. Andrew Mathieson and Mr. M. J. Jones of the Leicester County 
Museum, and the staff of the Clarks Shoe Museum, Street, Somerset. Figures 
10 and 11 were kindly provided by the Institute of Geological Sciences, 
London. 

I thank Mrs. Janet Clarke and Mrs. Lynda Spicer, rom, for their secre- 
tarial help; Mrs. Sophie Foray, rom, for the drawings; Mr. Leighton Warren, 
Photography Department, rom, for photographs; Mrs. Erica Wolfe, rom, 
for German translations and Miss E. Dowie of the rom Library for assistance 
with literature searches. For reading the manuscript and making many help- 
ful suggestions I thank Dr. Allan Baker of the Department of Ornithology, 
ROM, Dr. R. V. Melville of the Institute of Geological Sciences, London, 
and Dr. C. S. Churcher of the University of Toronto. 



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