|Ingentia prima by Jorge Gonzalez|
First, let’s get some terminology out of the way. I’m going to use the phrase “prosauropod” as a term of convenience here to mean “every non-sauropod sauropodomorph.” This is more or less how the term has been used for a very long time—animals like Plateosaurus, Massospondylus, and Anchisaurus were referred to as “prosauropods” in a paraphyletic sense but the paraphyletic nature of the group is well-established. “Prosauropods” constitute a loose amalgamation of taxa that form a stepwise sequence from Buriolestes to actual basal sauropods like Vulcanodon. The actual line in the sand between “derived prosauropod” and “basal sauropod” is becoming somewhat difficult to draw—a few key taxa (Aardonyx, Melanosaurus, Vulcanodon) move between these branches with some fluidity from phylogeny to phylogeny which we’ll get to.
Let’s talk about lessemsaurids.
Along with its neighbors, Riojasaurus incertus and Coloradisaurus brevis, Lessemsaurus sauropoides was named by Bonaparte for material found in the Late Triassic Los Colorados Formation of Argentina (Bonaparte, 1999). The holotype of the latter consists merely of eight pre-sacral neural arches which isn’t a lot to go on but who am I to judge? Bonaparte referred more material to Lessemsaurus in that initial description, and a 2007 redescription of the taxon (Pol & Powell) includes a scapula, humerus, radius & ulna, some finger bones, a largely-complete right half of the pelvis, a femur, both tibiae, the astragalus, and several foot bones.
Bonaparte originally referred Lessemsaurus to the Melanorosauridae which, in 1999, may or may not have been a monophyletic group seeing as the very meaning of “prosauropod” was in flux. Yates & Kitching (2003) noted that Lessemsaurus and their new taxon, Antetonitrus ingenipes, shared a distally flared dorsal neural spine. Pol & Powell (2007), thanks to the new Lessemsaurus material, find additional characters that match it and Antetonitrus but are absent in more traditional “prosauropods,” indicating that these two taxa are closer to Sauropoda than other prosauropods.
|The known material of Antetonitrus|
Yates & Kitching find Antetonitrus between Melanorosaurus and Isanosaurus in a stepwise series towards Eusauropoda; in their phylogeny, Anchisaurus is the most basal true sauropod. One thing that made me smile is that the authors peg Azendohsaurus laarousii and two unnamed forms from Madagascar as “fragmentary basal sauropodomorphs of…indeterminate systematic position.” In fact, we would later find out that they’re not dinosaurs at all!
McPhee et al. (2014) later published a lengthy paper detailing the osteology of Antetonitrus and further tested its phylogenetic position. Notably, they find that three individuals—not two—are present in the remains. The authors more or less torpedo the notion of a monophyletic Prosauropoda, although some small monophyletic groups remain (Plateosauridae, Massospondylidae). To deal with Lessmsaurus and Antetonitrus, McPhee et al. reintroduce an older phylogenetic node called Sauropodiformes, which for our purposes essentially separates the traditional “prosauropods” from the “basal sauropods” of Yates & Kitching (Anchisaurus, Melonosaurus) from the true Sauropoda—here inclusive of Gongxianosaurus, Vulcanodon, Isanosaurus, and then Eusauropoda. Their basalmost eusauropod is Tazoudasaurus.
I like this approach, which recognizes the paraphyly of non-sauropod sauropodomorphs while still recognizing that the larger, more sauropod-like taxa occupied a sort of (here we go) transitional morphology, yet this “middle of the road” group is not, in McPhee et al.’s phylogeny, monophyletic.
They write that:
…the basal sauropodiform hand represents a functional trade-off between the need to retain manual dexterity and mobility whilst providing an important auxiliary role in resisting large loading stresses…” and that Antetonitrus is a critical taxon in understanding this transition. Antetonitrus and its closest relatives retained a grasping forelimb and probably the ability to switch to bipedal locomotion when the need arose (like running away from rauisuchids).
Then, just about a month ago, a new challenger entered the ring: Ingentia prima, a very large non-sauropod sauropodiform (Apaldetti et el. 2018) along with three new specimens of Lessemsaurus which together demonstrate that non-sauropod sauropodiforms grew very large very fast, reaching sizes that true, blue sauropods wouldn’t achieve until well into the Jurassic. Based on histologic sampling, Apaldetti et al. propose that Lessemsaurus weighed upwards of seven tons as an adult, which is even larger than basal true sauropods.*
|Skeletal diagram and selected bones from Igentia|
The group has an interesting growth strategy: while normal sauropods grew at a fast, but constant rate towards adulthood, these lessemsaurids had “cyclical and remarkably high growth rates.” In other words, between the lines of arrested growth, they did a LOT of growing. They also have pneumatic cervical and (in Antetonitrus) dorsal neural arches, and thus had evolved at least the beginnings of an avian-style respiratory system, or at least the beginnings of it. Interestingly, lessemsaurids did not have very long necks and may not have engaged in the sort of bulk feeding that true sauropods specialized in. While there is no skull material known for lessemsaurids, close relative Aardonyx celestae (Yates et al. 2010) has a triangular snout in dorsal view, indicating that it was more selective in its grazing than true sauropods (but read on).
|Skeletal and bones of Ludumahadi|
|The skull of potentially close relative Aardonyx|
Apaldetti et al. (2018) note that, in Ingentia at least, the forelimbs were incapable of an erect posture and the hand could not be fully pronated. It's clear that Antetonitrus had a divergent thumb, so grasping was still in the cards. And it's entirely possible that these taxa had different hand morphologies: Antetonitrus is the smallest (but still large) but Ludumahadi is largest and may have a more sauropod-like hand--independently derived, of course.
|A skeletal of Aardonyx, who seems to be bipedal|
Finally, I wonder what triggered gigantism in this group in the first place. One of the benefits of getting larger is that your digestive system also gets larger, allowing you to better handle bulk-feeding, which is a theory applied to sauropods and why they got bigger. Of course, another huge benefit to becoming large is that once you weigh more than the local predators, you don’t have to worry that much about being preyed upon. The Los Colorados Formation is home to Zupaysaurus and Powellvenator (coelophysoids) as well as Riojasuchus (a mid-sized ornithosuchid). Given the general agreement that they weren't bulk-feeding, I’m more inclined to chalk lessemsaurid gigantism up to predator avoidance. This is indirectly supported by their high growth rate—get bigger faster before something eats you!
There's a lot more I could say about this, including talking about Aardonyx, another of these borderline taxa that tend to move around the tree, but this post has already gotten out of hand. The moral of the story is that sauropods weren't the only sauropodomorphs to get huge--non-sauropod sauropodiforms did it, too!
*The authors don’t actually calculate the weight of Igentia, which is weird, unless I missed it.