Left: T. testudinalis from Runswick Bay, Yorkshire. 1909. © J. Middleton, Scarborough Museums.
Right: Mean temperatures in central England 1800 – 2019. Crown Copyright, Open Government Licence V2.
T. testudinalis advanced south from Scotland by over 220 km in the cold period c. 1855 – c.1920 when the ten year running mean temperature in central England was always below the mean temperature of 1961 – 1990. From c.1920 to the late 1980s the ten year running mean temperature rose to fluctuate around the level of the 1961- 1990 mean. In this period, records of T. testudinalis continued to be made so it seems to have maintained its expanded range, though no specimens or images have been viewed in this inquiry, so some may be misidentified T. virginea . From the late 1980s to 2019 the ten year running mean temperature has risen to unprecedented heights well above the 1961-1990 mean and there has been a marked reduction of reliable records of T. testudinalis in north-east England and elsewhere.
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig