Arthropod trackways – there is no direct evidence for myriapods prior to the Silurian. However, indirect evidence in the form of trackways from the Early Ordovician suggest the existence of amphibious, myriapod-like arthropods. The makers of the tracks in the above image were among the first animals to start colonising the terrestrial realm. The trackways in A and B are proposed to have been made my Euthycarcinoidea (not myriapods, but likely closely related), however C may have been made my a myriapod (but also could have been made by Euthycarcinoidea).
Synapta maculata – also known as the snake cucumber. Despite looking like a snake, it is in fact a giant (or at least very long) sea cucumber. It can grow up to 3 meters long and lives in the Indo-Pacific Ocean. Sea cucumbers are pretty famous for ejecting their internal organs to scare off or distract predators, however S. maculata instead just splits in two and re-grows the rest of its body at a later date.
Titanomyrma – a Paleogene ant the size of a hummingbird. Well, the queen could grow to the size of a hummingbird, i.e. around 6 cm. Worker ants of this genus are yet to be discovered in full. Members of this genus have been found in the Green River Formation in Wyoming and the Messel Shales of Germany.
Saurophthirus – at 2.5 cm long, Saurophthirus is not actually very big. But when you realise that Saurophthirus was a flea (!) from the Cretaceous that fed on the blood of dinosaurs and pterosaurs… it kind of changes your perspective. Saurophthirus was the largest flea to have ever existed (for reference, extant fleas can grow up to 2.5 millimetres).Although I don’t know an estimate for how high they could jump, their long legs suggest that like their extant descendants, they were also very good jumpers.
Inoceramus – an extinct genus of bivalves that originated in the Jurassic, within which is the subgenus, Platyceramus, aka. the largest bivalves of all time. They could reach up to 3 m wide, meaning they could grow considerably larger than the giant clam (Tridacna) which is know to grow up to 1.14 m.
As a disclaimer: molluscan cladistics is a rabbit hole that we will probably never escape from. It get’s messy. So the very short answer would be: who knows?
But that isn’t helpful. So instead I will say: it’s not proposing anything that hasn’t already been proposed, and you could definitely find a paper with evidence (not necessarily good evidence) for why that is a very reasonable cladogram. It is very much based on morphology, and we have new genomic data that suggest differently, so I would say it is quite outdated.
I have never heard “Eumollusca” as a clade before, but it is the same as Testaria, which I have seen proposed. It was formed based on the idea that aplacophora (caudofoveata) don’t have shells, while the rest of the molluscs do. But that isn’t really supported by genomic data. It would be better if caudofoveata and polyplacophora were put into their own clade, Aculifera, besides Conchifera.
The way they divided up Conchifera is also questionable, but not unreasonable. It’s pretty common to see cepahlopoda, scaphopoda, gastropoda and bivalvia put into their own clade, excluding monoplacophora. Likewise, a clade consisting of cephalopoda and gastropoda is supported by features such as their well defined head region (cephalisation). However, there is growing evidence that monoplacophora and cephalopoda are sister taxa – firstly this has been suggested by some genomic studies, but also by the fossil record that suggests they had very similar shell strucutres.
The above cladogram currently seems to be the most well supported, e.g. Smith et al. 2011 and Kocot et al. 2011 (both using a genomic analysis). Kocot et al. further proposed Pleistomollusca (labelled), consisting of gastropoda and bivalvia, while Smith et al. instead proposed that scaphopoda and gastropoda should form their own clade.
We can mess things up a bit more if we consider a study by Stöger et al. (2013) that instead came up with this…
Unfortunately, analysing different genes gets you different results. Then I guess we have to ask which genes are going to get you the most accurate results? At that point I have absolutely no idea. I have done very little genomics and wouldn’t even know how to start assessing the accuracy of different methods.
I struggled to find a giant for the Triassic, so thought instead I would explain why there were no giants in this period. It’s thanks to a phenomenon known as the Lilliput effect. Following a mass extinction or disturbance event, we see a reduction in the relative body size of the surviving organisms. In some cases, larger species were just more likely to go extinct than smaller species. However, it is also observed that animals just start growing smaller (e.g. the Permian extinction was the result of global warming. Global warming = ocean acidification = less carbonate ions in the water = calcifying organisms, i.e. gastropods and bivalves, can’t grow as big as they could before). This phenomena is not just limited to invertebrates. For example, land mammals saw a reduction in size during the Paleocene Eocene Thermal Maximum.
So then why do we see giants during other periods following mass extinctions? The Triassic begins after the end-Permian mass extinction, the largest ever mass extinction which resulted in a species loss of up to 90%. While some groups of taxa were certainly affected more than others, it was pretty devastating across the board. Meaning there were no groups that came out relatively unscathed and the Lilliput effect is observed in a huge range of species.
Meganeuropsis – a Permian griffinfly with a wingspan of ~70 cm, and the largest known insect of all time. As kind of discussed here, it is thought that insects were able to grow so large in the Permian due to the increased oxygen in the atmosphere. Earth Archives have a great article about them (with some cool artwork).
Arthropleura – a genus of millipedes from the Carboniferous. These guys are some of the largest land invertebrates to have ever existed, and likely had no predators. Despite their size, Arthropleura were lovely herbivores.
I am recycling an old post again because Arthropleura is arguably the coolest of the Carboniferous invertebrates, and because I have been thinking about myriapods ever since this morning when I was asked to do myriapod evolution next and I am now really excited for that!
Image Credit: S. Braddy & co. 2008 – a. Jaekelopterus rhenaniae, b. trilobite, Isotelus rex, c. dragonfly, Meganeura monyi, d. millipede, Arthropleura armata, e. chelicera of J. rhenaniae. Scale bar a–d. 50 cm e. 10 cm.