Genus Megerlia King, 1850

Remarks: The validity of the three species Megerlia truncata, M. monstruosa and M. echinata has been a matter of debate for the last nearly 200 years, with many of the greatest brachiopodologists in history taking opposite views. As the issue is still not settled, I feel it is necessary to give an overview of the history of these species. Already, when erecting M. monstruosa, Scacchi (1833) admitted doubt, whether the specimen of his new species in reality was the species M. truncata, which just had shaped itself after the substrate. This opinion of synonymy was shared among others by Costa, Monterosato and Jeffreys based on the internal features, though Jeffreys kept the species as a variety of M. truncata (see: Costa 1838; Monterosato 1872; Jeffreys 1878). Davidson (1887) followed the lead of the others but observed that all specimens he had encountered of M. monstruosa differed in a uniform way from M. truncata, why he chose to keep it as a variety. Subsequently, Fischer & OEhlert (1890) erected a very similar species, M. echinata, mainly differing by having minute spines on the surface. They then followed up by restoring M. monstruosa to species level arguing that while M. truncata often showed deformation due to confinement of substrate, the degree of deformation in shells of M. monstruosa was so extreme and consistent, like the mode of convexity; ornamental characteristics; narrower shell; pedicle opening moved into dorsal valve; the smaller interarea; and the cardinal area on the inside, that they were species-specific (Fischer & OEhlert 1891). Dall (1919 and 1920) found the shared feature of the pedicle opening allocated to the dorsal valve warranted, that M. echinata and M. monstrousa with M. monstruosa as type species were moved into a new genus Pantellaria. Based on observed variability within Megerlia truncata, Thomson (1927) questioned the validity of the suggested generic differences as more than species-level differences. Atkins (1961a) followed up with a study on M. echinata. There she did not find support for separating this species, but lacking specimens for the study of M. monstruosa, did not suggest transferring that back as well. Most accepted the transfer, though some maintained Pantellaria for M. monstruosa long after (e.g.: Cooper 1981a, 1982; Zezina 2010, 2014). While noting M. monstruosa externally closely resembled species of the genus Platidia, Logan (1979) again put M. monstruosa into synonymy with M. truncata, though not providing a discussion on why. Comparing the specimens of M. echinata from Atkins (1961a) with those of M. monstruosa used by Davidson (1887), Brunton (1988) disagreed on this synonymy and argued that M. monstruosa and M. echinata were synonymous though distinct from M. truncata. He thus placed them under the senior name M. monstruosa, an opinion which was followed by Anadon (1994). His study seems to be rather unique in that it directly compared the two species and did not just give a discussion on how each of them differed from M. truncata.

Just two years later, Bitner (1990) published a study on 13–16 million-year-old Middle Miocene Polish fossils. Though she did not find specimens fully resembling M. echinata and M. monstruosa among her more than 2500 specimens, she did have intermediate specimens and thus concluded, that all three species were one and the same. In general, I hesitate to put too much weight on using the shell morphology of so old specimens as a proof of species synonymy of living brachiopods. This is because 15 million years is a very long time for a species, and both the morphology, and the degree of morphological variability, can change significantly over such a time span, as well as that species can split up in a shorter time (e.g.: Sheldon 1987; Wei & Kennett 1988). Nonetheless, this publication became an important basis for changing the major opinion towards the existence of only one species. Interestingly, in a simultaneous study on nearly 1000 specimens from the same Polish deposits, Barczyk & Barczyk (1990) came to the opposite conclusion, that they had two distinct species, M. truncata and M. monstruosa. Barczyk & Barczyk (1990) found the differences so distinct and ontogenetic stable, that they preferred to follow the opinion of generic division of the two species.

With supporting observations of recent morphologically intermediate specimens, Logan et al. (2004) agreed with Bitner (1990) that evidence points to M. monstruosa being conspecific with M. truncata, though they recognized it as a variety and accepted M. echinata as a valid species. This opinion on synonymy was followed by Anadon et al. (2022), Logan (2007), and Logan et al. (2007, 2008). Others, like Alvarez & Emig (2005), Hiller et al. (2008), Alvarez (2016) and Emig (2016), were more inclined to agree with Bitner (1990) in that all three species are synonymous. Trying to give a better basis for comparison, Simon et al. (2016) provided SEM pictures of the holotype of M. echinata together with a discussion. They found the nature of the micro-ornament differed from that of M. truncata, why they concluded M. echinata is a valid species distinct from M. truncata.

At present, it seems that the greater majority consider both M. monstruosa and M. echinata synonymous with M. truncata, while a large minority consider M. echinata a valid species.

The division in the present work is based on the following reasoning. On a general basis when assuming two species are synonymous, it would be expected that the various features show more intermediate forms as of the type form of the least common form group. Similarly, it would seem unlikely to encounter a relatively consistent combination of seemingly uncorrelated features differentiating two subgroups within the species if no external barriers keep populations separate over an extended time.

Going through the available material of the genus, there turned out to be two distinct form groups for which I did not manage to find intermediates among the probable nearly 1000 specimens I have seen. The biggest group contained specimens rather conservatively biconvex and unisulcate, with the major part of the pedicle opening in the ventral valve, and with both valves exhibiting dense, finely costellate ribs as well as low tubercles. Deformation in these specimens appeared only to occur in cases where the specimens had been cramped by substratum or damaged during life. This form group has the typical shape of M. truncata.

The smallest group contained the specimens with dorsal valve typically nearly plane to concave, shaping itself after the substratum and rarely showing any hint of ornament other than growth lines; a convex ventral valve with sparse and uneven, weak costellae with scattered tubercles, as well as variably developed minute and alveolate thorns and spines; and a pedicle opening moved almost entirely into the dorsal valve in mature specimens. Specimens still attached to their substrate, showed that the odd shape was not due to confined space or shell damage during life. The shape and the minute spines place this form group as M. echinata. I was unable to find any evidence of external reasons for the general shape of the specimens of M. echinata. I thus find myself in support of the opinion, that M. echinata is not synonymous with M. truncata, and therefore chose to keep them separate in this publication.

Another observation from this study was how unevenly distributed the minute thorns and spines were developed on the ventral valve of many of the examined specimens in the form group of M. echinata. These spines were evenly and densely distributed on some valves, while only present on parts of valves or nearly lacking on others. Specimens lacking these spines fit in both shape and ornamentation the description of M. monstruosa and I therefore feel inclined to think that Brunton (1988) and Anadon (1994) were right in that the two species are synonymous rather than M. monstruosa with M. truncata. However, I here realise there are strong opinions to the contrary, and as I have not seen the type specimen of M. monstruosa, I leave this question for others to clarify. If, however, the two species turn out to be synonymous, the name M. monstruosa is the senior synonym.