Northern limits of distribution
Several museums hold specimens of T. testudinalis from the northernmost coasts of the land masses encircling the Arctic Ocean, including those which are icebound for part of the year. Although the southern limit of distribution is receding northwards, there seems little unoccupied coast for it to advance onto northwards with global warming. It might already extend undetected below the sea ice to the North Pole. Some most-northerly museum-records on GBIF are:
North Alaska 71.3° N (1900);
Devon Island, Canada 76.6° N (1962);
Northern Greenland 76.2° N (1894);
North-west Iceland 65.88° N (2019);
Svalbard 77.7° N (undated);
Nenets Region, northern Russia 69.8° N (1875).
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig