One plus one is more than two

In memory of Jeff Mulroy

Coroniceras lyra (left, 85 mm) and Arnioceras ceratitoides (right, 55 mm)

This is one of the pieces I bought from the collection of Jeff Mulroy, who sadly passed away in October 2018. You will see a few more specimen from Jeff’s collection in the next couple of posts…
This specific one came to me through Mike Marshall, who had bought it from Jeff, re-prepped it and kindly sold it on to me; I do not know what it looked like initially, but I assume from what I’ve seen from Jeff’s prep work that Mike only had to further enhance the already good prep that Jeff had done a little. I guess it is either an erratic specimen from the Holderness coast or from the Redcar/Marske area where Jeff often collected, its origin is the lyra subzone of the semicostatum zone of the lower Lias.

Arnioceras ceratitoides, 55 mm and a small Coroniceras, 10 mm at the top

Arnioceras ceratitoides, keel view

As he sold it, Mike had quite correctly identified the larger 55 mm ammonite on the right as Arnioceras ceratitoides (QUENSTEDT,1848), a very finely ribbed species of Arnioceras which to that point I not had in my collection (or so I thought…) I’ve counted 45 ribs at 55 mm diameter, which is significantly higher than in all the other Arnioceras species I’ve described in In this specimen, there is no ribbing for about 1 cm, then radial ribbing until about 4 cm, then the ribs become concave, leaning a little forward at the venter.

The naming of this species is due to its supposedly reduced, ceratitic sutures, which I could not see on this ammonite due to its preservation (this will shortly be added – I’ve just bought another A. ceratitoides on eBay which has a nice suture).

Coroniceras lyra, 85 mm
Coroniceras keel view
Coroniceras keel view

The larger 85 mm specimen on the left was a little more difficult to identify, but I could see that is was not the Euagassiceras resupinatum that Mike had identified it as when selling it through his website. This ammonite has furrows on the side of a prominent keel, which Euagassiceras resupinatum lacks. Also this ammonite has more dense ribbing with 32 ribs at 7 cm, a Euagassiceras has about 23 ribs at 7 cm.

Euagassiceras resupinatum, 70 mm for comparison

Over the years I have entered all the ammonite literature that I could get hold of in pdf format into a full text database, in total more than 600 books and articles, which makes life researching ammonites so much easier…
So what I did is I entered “Arnioceras ceratitoides” in the search to see if there are any hints with which other ammonites this species co-occurs. I got about 40 matches, but the most interesting came from M.J. Simms chapter about Hock cliff in the Severn Basin section of the “British Lower Jurassic Stratigraphy” (see literature section below), where he mentions it co-occurring in Hock Cliff bed 28 with Coroniceras lyra HYATT, 1889. Another search in the database for pictures of Coroniceras lyra found a good match in Hyatt´s “Genesis of the Arietidae” (table 5) and various other articles/books, also under synonyms Paracoroniceras lyra (which probably is the most current name) and Coroniceras reynesi.

Coroniceras lyra, 26 mm

As for the smaller ammonites on this specimen, as the 26 mm one above,
I think they’re “just” juveniles of Coroniceras lyra (which is nice in its own way – it’s always good to know what different growth stages of a species look like).

As a bonus, the Coroniceras shows a few smaller ammonites (which I assume are Arnioceras as well) washed into the body chamber, and just visible inside the internal mould.

Small Coroniceras lyra (21 mm) and 2 Arnioceras ceratitoides (25 and 35 mm)

Now when I put back this little multi block into the drawer, I noticed another multi specimen that I had also acquired from Mike Marshall, but already in 2014.

The Arnioceras ammonites (35 and 25 mm) on this multi are also Arnioceras ceratitoides, and the small ammonite on the left (21 mm) is most also a Coroniceras lyra – compare the odd splitting of some of the ribs from a small node at the umbilicus into two ribs.

This specimen also has a fragment of a larger Arnioceras on the left, and a convenient keel view of an an Arnioceras (or fragment thereof) embedded perpendicular relative to the other ammonites, showing clearly the furrows beside the keel, which in addition to the high number of ribs/whorl is another characteristic of A. ceratitoides.

The two different species are interesting and rare in their own right, but their combination on these fascinating little multi blocks creates a context where the sum is greater than the parts.


Schlegelmilch, R.,Die Ammoniten des süddeutschen Lias,

2nd revised and extended edition, Stuttgart & New York 1992

SIMMS, M.J.,(2004), Hock Cliff, in : British Lower Jurassic
Stratigraphy, Geological Conservation Review Series, JNCC
Peterborough 2004, pp. 164-170

Hyatt, A. (1889) Genesis of the Arietidae, Smithsonian Contributions to Knowledge, No. 673, Smithsonian Institution, Washington DC