Pool frog Rana lessonae in Britain: past nativeness unresolved. On the verification of sub-fossil bone identification

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Pool frog Rana lessonae in Britain: past nativeness unresolved.

On the verification of sub-fossil bone identification.

Published 27th October 2005

Tom E S Langton and John A Burton

A report by:
Herpetofauna Consultants International Ltd.

Triton House



UK Suffolk IP19 9AE
01986 784596 t.langt@virgin.net


A review has been conducted on Green frog sub-fossil bone identification, as reported in recent publications, notably;

Gleed Owen C P (2000) Subfossil records of Rana cf. lessonae, Rana arvalis and Rana cf. dalmatina from Middle Saxon (c.600-950 AD) deposits in eastern England: evidence for native status. Amphibia-Reptilia 21: 57-65.
The opinion of the world expert and european authority Dr Gottfried Boehme is quoted in the above paper to confirm the authors identification of a Pool frog sub-fossil bone and appears to have been misrepresented.
The paper gives the impression that the identification of a Pool frog Rana cf lessonae subfossil type ilium (the “hip” bone with a socket for the top of the upper leg bone) had been confirmed by Dr Boehme and is a “prudent” identification. It is revealed in this report that the specimen shown to Dr Boehme by the author (Gleed Owen) was only narrowed down by Dr Boehme to the bone being probably of either an Edible frog or Pool frog type.
Further, the statement in the above paper that Dr Boehme agreed to the validity of Gleed Owen’s method of visual appraisal of the shapes and textures of ilial morphology is refuted. It has not been shown to be a proven and verifiable technique.
The identification methods of Gleed Owen (2000) are therefore not “prudent” as indicated in the text of the paper and cannot be trusted. Re-examination of relevant material by an independent expert is strongly recommended, with an appropriate peer review.
As a consequence of the questionable identification process, subsequent published scientific and government documentation incorrectly suggests that Pool frog was once a native British species e.g. Beebee et al (2005) and Buckley and Foster (2005). The claim of confirmation of identification of pool frog sub-fossil bones has been used extensively in 2005 and as the main (only) physical evidence to argue that there is evidence that Pool frog was a natural post-glacial coloniser of England that survived until modern times (Beebee et al 2005) and in justification of substantial nature conservation funding for, and including the release of Pool frogs in Norfolk in August 2005 by English Nature and others.
The appropriate conclusion for available material is that bones in mid-Saxon British archaeological sites indicate that frogs from the Green frog complex are represented but that it has not been shown which of the three types most likely: Edible (Rana esculanta), Pool frog (Rana lessonae) and Marsh frog (Rana ridibunda) were present, nor if their origin was natural or as the result of human introduction and it is noted that bones located are all from sites associated with human settlement.
It is considered that, in the present state of knowledge, cranial bones from a non-human site, and from pre-Roman deposits would be needed to confirm the native status of any Green frog species in Britain. If one or more Green frogs were present after the last ice age there should be ample material in remaining natural sites for this matter to be determined.


The GReen Frog Information Programme (GRIP) was established by HCI in 2004, following concern at the announcement in BBC Wildlife Magazine that English Nature was promoting the release of Pool frog Rana lessonae into the wild in Norfolk. A number of private landowners in Eastern England have combined to attempt to clarify the scientific basis behind the reporting. The aims of GRIP are:

1. To collate and assess information on Green frogs in Britain with respect to recent views expressed that the reports of Green frog in Fowlmere, Cambridgshire and the Thompson Common area of Norfolk and elsewhere (as sub-fossils) indicates Pool frog Rana lessonae to have been a recent native to England.

2. To advise and represent landowners with respect to any change or future proposed change to guidelines, listings or legislation concerning Green frogs in Britain.

3. To assist in developing a forward-looking strategy for Green frogs in Britain.

This report follows the GRIP report (Burton and Langton 2005) that indicated serious and multiple inconsistencies in an English Nature research report. This English Nature report had suggested that archival/historic evidence suggests that two frog colonies first recorded in the nineteenth century (now extinct) in East Anglia were Pool frog colonies of natural origin. This was shown in the GRIP report to be a biased summary of the available evidence, that the conclusions of the report cannot be justified and require re-evaluation.


In August 2005 English Nature reported the release of 170 Pool frogs at a site near Thetford, Norfolk, captured in Sweden. They were released in deepened pond areas including old pingo formations within a commercial tree plantation, close to a former and well-known Green frog site at Thompson Common in Norfolk. This action has made the investigations into the evidence relating to the identification of sub-fossil bones even more relevant as substantial nature conservation resources will continue to be required if the project is extended

For this review, detailed aspects of bone identification are addressed. Published and unpublished material was evaluated and sources of information for the key papers checked. The key publication is: Gleed Owen C P (2000) Subfossil records of Rana cf. lessonae, Rana arvalis and Rana cf. dalmatina from Middle Saxon (c.600-950 AD) deposits in eastern England: evidence for native status. Amphibia-Reptilia 21: 57-65.

This paper stood out for two reasons. First, it was claimed that it is possible to definitely distinguish Pool frog from Edible frog using subtle differences in shape of small parts of the ilium, (the paired pelvic or “hip” bone, including the hip socket). Secondly, the methods involved a subjective description of shapes and angles and relied on the expert opinion of a third party, who was not an author of the paper, to confirm the identity of a single, incomplete and degraded specimen.

The claims were of concern because it is known that bone shapes of animal species can vary quite rapidly over periods of as short as a few hundred years, particularly in newly introduced (small mammals in the Hebrides) or recolonising species.

Prior to the publication of Gleed Owen (Op cit), the published literature indicates that skull bones (fronto-parietals) were the only reliable way of distinguishing between the main types of Green frog, and as far as we can ascertain, no subsequent authors have used Gleed-Owen’s methods.

The identification of a second Pool frog ilium in 2001 (Beebee et al 2005) is also considered in this report.


Gleed Owen (2000) has been evaluated for its content in respect of material that is described as being Rana cf lessonae. Consideration of the identification of the two other species is not considered in this report.

As a result of investigations into the unusual presentation of identifications, interviews were conducted with the expert authority quoted; Dr Gottfried Boehme (retired) of the Naturhistorisches Forschungsinstitut Musuem fur Naturkunde, Berlin. Work by Dr Boehme is the basis for consideration of Green frog bone morphology Boehme G (1977) Boehme G and Gunther R (1979).

Dr Boehme was interviewed by telephone on four occasions and following this a letter was written by him for publication in this report and a suitable journal.


Gleed Owen (2000) includes consideration of a mid Saxon Green frog bone fragment (dated ca 880-1040 AD) found in a sill-beam slot, that was located in the foundations of a house at Chopdick Drove, Gosberton, near Spalding, Lincolnshire. The site includes rectangular structures apparently of unknown use, with surrounding ditches and pits.

Gleed Owen developed his own diagnostic criteria for Green frog ilial bones. He then blind-tested on a random selection of 48 water frog ilia from a museum collection. How the randomness was calculated or decided is not specified. The method is reported to involve the discarding of ‘ambiguous’ specimens – it is not clear how these are judged, and it is not clear in the supporting research reports on the subject. Of the 29 specimens not considered ambiguous, it is not clear how they divided between the three types of Green frog but the inference is that the sample size is very small for each.

The paper refers to Gleed Owen consulting Dr Gottfried Boehme, (presumably showing him the fragment, or pictures of the fragment at some point in 1998/99) and the subsequent conclusions drawn on the Gosberton Green frog bone fragment, in the paper are as follows:

In summary, the combination of a high, robust, laterally produced tuber, a deep concavity below the ala and the tuber, a thick junctura with a low d/t ratio, and an upstanding dorsal acetabular margin points towards this specimen being R.lessonae. However, G Boehme has examined this specimen and although he is certain that it belongs to either R.lessonae or R.esculenta, he preferred not to discount the possibility that R.esculenta ilial variation could overlap with that of R.lessonae. Thus as there is a slight element of doubt in this diagnosis, assignment to R.cf.lessonae would be more prudent. “

Dr Boehme, when contacted as a part of this research indicated that he was not aware of the paper and a copy was sent to him. Following this on the second contact he said that he had checked his files and found that he had been sent a copy at some point but had not had time to read it as it had not been his current area of research interest. He then read the paper and consulted with his colleague on Green frog identification Dr R Gunter. He then provided a letter commenting on the Gleed Owen (2000) paper. The text of the letter is reproduced as follows:

Statement from Dr Gottfried Boehme concerning Green frog identification.

Distinguishing Green frogs from ilia

The discovery published by C. Gleed Owen of Rana lessonae (pool frog) from Chopdike Drove, Gosberton, Lincolnshire published in Amphibia Reptilia and quoting my opinion requires qualification. In my opinion the range of the variations in the features of the ilium of Rana esculenta is wide and the features of Rana lessonae come within it. Differentiating between brown frogs and green frogs based on the proportions of the ilio-ischiadica is essentially safe but distinguishing between green frogs is difficult and prone to error. Although the Chopdike Drove find shows ilium features which can occur with Rana lessonae, I do not think that a single fragmented ilium is adequate for identification of this animal. The fronto-parietalia of Rana lessonae however do have clear features, which make it possible to effect a fairly good differentiation between species and hybrid and this bone should be used. Therefore it is not prudent, as suggested in the paper to refer to the Gosberton bone as coming from Rana cf. lessonae as it is possible that it could be from the hybrid Rana kl. esculenta.
Further, the unpublished manuscript from Beebee et al (2005) shows a further bone from Ely that has also been identified from Gleed Owens reported 'blind testing' from the Berlin collection and says that this enables precise identification. I can confirm that I made my facilities available and that the individual characteristics given by Chris Gleed-Owen for differentiation are more numerous than developed by me. Although it is possible that this method can separate the species from the hybrid this method has not been published and is not something that I can confirm.
The only way to confirm the findings with a degree of certainty would be to develop a reference collection of green frogs of all types, age and sex and to record bone morphology according to clear criteria. This would demonstrate the degree of overlap of characteristics and the situations where definite identification can be made. This would enable a Key to be developed for anyone to check existing and future material. Otherwise in my view the separation of species will always be questionable as is its use in informing applied nature conservation projects.

Dr G Boehme March 2005

Beebee et al (2005) repeats the assertion (now shown above to be incorrect) that Boehme agreed with the identification of the Gosberton bone as Rana cf lessonae. It gives a further description for distinguishing Pool frog ilia from those of Marsh and Edible frog as follows:

In R lessonae, the tuber is narrower antero-dorsally, sometimes more sinuous and always more laterally produced than in R ridibunda. The posterior margin of the tuber is also steeper in R lessonae and the dorsal edge of the vexillum tends to be straight as opposed to convex in R ridibunda. R esculenta is intermediate between R lessonae and R ridibunda in ilial morphology, but can generally be separated from either species. The shape of the corpus and the angles of its margins, in combination with tuber and vexillum shape, allow the distinction of R lessonae from R esculenta in most cases.”

The contested identification claiming Pool frog bones is also repeated in the English Nature report Buckley and Foster (2005)


The published and unpublished literature shows that anything other than very experienced workers can get amphibian and reptile bone identifications wrong. This is due to the unknown variation and similarity of appearances of bones within and between species and the range of levels of preservation of sub-fossil finds.

To understand the full variations between species using bones that have not been shown to separate in a clear cut way (in Green frogs the head bones) would require a modern bone reference collection from a range of populations across the ranges of the species concerned, comprising a range of adult sizes/sexes and for the bones to be subject to accurate measurement from identifiable points.

Without this, the trends reported by Gleed Owen (2000) and in Beebee et al (2005) are as likely to be an artefact of materials available to those studies as to be a genuine methodology and are not robust. Any blind-testing of new proposed methods would need to be done by suitably experienced archaeozoological workers familiar with general small animal bone morphology but without prior experience of the material.

Current archaeozoological research on Green frog bones in England is inconclusive. It is most likely to indicate that between one and three frog of the types Edible, Pool and Marsh frog may have been present in England, at least in places close to human settlements from around 850 AD and there is evidence of this from other sites in England at later dates, including London (Armitage and West 1985). There is as yet no firm evidence as to whether Green frogs were present naturally or introduced or even of both origins but lack of records before this date, despite numerous and multiple frog bone finds, suggests that introduction of Green frogs by humans is as likely as past-native origin and arguably more so. Further, there is no evidence for archeozoological sites of Green frogs existing away from human settlements.

Evidence for and accounts of frog-leg meat consumption by humans in Britain has not been collated, but evidence does exist. Green frogs would have been a significant seasonal food item for humans in northern Europe before modern farming techniques and the native grass frog is likely also to have been consumed to some degree. Movement of commodities including live animals from mainland Europe to Britain greatly increased in Roman times and became very substantial by the 11th century. Importation at or after the time of the introduction of other food animals to England, such as Rabbit Orictolagus cuniculus and Brown hare Lepus europaeus is still the most likely period for the origin of Green frogs in England in the absence of pre-Roman material although introduction before these periods remains possible.

If Green frogs occurring in England are the result of introductions then all possible sources and species should be considered, in view of the extensive maritime trade routes ranging from the Baltic to the Mediterranean, for pre-Roman, through Roman and on to the 20th Century.

This report considers that archaeozoological aspects of the case for considering Pool frog to be a past native species are contentious, and that their recent release in Norfolk is therefore premature. The bones in question are possibly of “Green frog” bones, but their identification to any particular species, and their consequent identification to a native species worthy of reintroduction, is considered inaccurate. It is considered that, in the present state of knowledge, cranial bones from a non-human site, and from pre-Roman deposits (preferably several), are needed in any attempt to confirm the post-glacial native status of any Green frog species in Britain. Further, given known changes over time of bone morphology care must be taken to ensure that methods and findings are applicable to any sub fossil and archaeological material.


Armitage P L and West B with contributions from B T Clarke, T Dyson, M F W Festing and A Locker (1985) Faunal evidence from a late medieval garden well of the Greyfriars, London. Transactions of the London & Middlesex Archaeological Society Vol 36 1985/1987

Beebee T J C J Buckley, I Evans, J P Foster, A H Gent, C P Gleed Owen, G Kelly G, Rowe, C Snell, J T Wycherley, and I Zeisset (2005). Neglected native or undesirable alien? Resolution of a conservation dilema concerning the pool frog Rana lessonae Biodiversity and Conservation 14: 1607-1626.
Boehme G (1977) Zur Bestimmung quartarer Anuren Europas an Hand von Skelettelementen. Wissenschaftliche Zeitschrift Der Humbolt-Universitat Zu Berlin, Mathematik-Naturwissenschaft, 26, 283-300
Boehme G and Gunther R (1979) Osteological studies in the European Water frogs Rana ridibunda, Rana lessonae and Rana “esculenta” (Anura, Ranidae). Mitt. Zool. Mus. Berlin, 55, 203-215
Buckley J and Foster J (Eds) (2005) Reintroduction strategy for the pool frog Rana lessonae in England. English Nature Research Reports No. 642 Peterborough
Burton J A and Langton T E S (2005) On the approach to investigating the historic status of the Green/Water frog Rana lessonae in England using archive material, with particular reference to English Nature funded Research Reports and proposals to release Pool frogs in the wild in England. 1st report of the Green Frog Information Programme (GRIP). Herpetofauna Consultants International. Bramfield.


Dr Derek Yalden is thanked for comments and text amendments. Barry Clarke (Herpetology section Natural History Museum, London) provided additional advice and assistance.

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