Some bones of holotype of Maip macrothorax have been previously described by Novas et al.3, thus we refer to this paper for such materials. The dorsal centrum originally described as belonging to a “posterior dorsal” is here reassigned to dorsal 13th.
Based on comparisons with the complete cervico-dorsal sequence preserved in a juvenile specimen of Megaraptor namunhuaiquii10, and holotypes of Aerosteon riocoloradensis14,33, Murusraptor barrosaensis12,32, and Tratayenia rosalesi19, the cervico-dorsal series of Maip macrothorax is represented by the axis (Fig. 2), D2 through D7, D9, D10 or D11, D12, and D13 (Fig. 3-8). Only three caudals are currently known for Maip macrothorax, corresponding to the proximal third of the tail, which can be compared with proximal caudals of Megaraptor namunhuaiquii10, Orkoraptor burkei34, and Aerosteon riocoloradensis14,33. Holotype of Maip macrothorax also preserves several ribs tentatively referred as cervicals 5, 7 and 8 (Fig. 10), and dorsals 1, 2, and 6, as well as many other indeterminate rib fragments (Fig. 11). Many lateral and medial gastral elements have been recovered, probably belonging to the anterior half of the chest (Fig. 12). As it is observed in other megaraptorids, Maip macrothorax exhibits proportionately low cervicals (as represented in this case for the axis) as compared with posterior dorsals, which are twice the height of anterior cervicals. The axial elements available for Maip expand the knowledge on axial anatomy in Megaraptoridae, offering novel anatomical information on axis, anterior dorsal series, cervical and dorsal ribs, anterior medial and lateral gastral elements, which remain unknown in other megaraptorids.
Axis: The axis is almost complete (Fig. 2), only lacks its right postzygapophysis and both prezygapophyses. In lateral view (Fig. 2 A), it is taller than long, being the neural arch almost two times the height of the centrum. The neural spine is low but robust, being less than half of the height of the neural arch. The intercentrum exhibits a rugose texture; it is placed below (not anteriorly) to the pleurocentrum, and immediately ventral to the parapophyses. The intercentrum is subtriangular in lateral view, and shows a moderate laterodorsal projection. Anteriorly, the intercentrum is crescent-shaped and anteriorly concave; it is subtriangular in contour in ventral view. The pleurocentrum shows an irregular anterior surface, the odontoid process being damaged. The pleurocentrum is squared-shaped in lateral view, being slightly taller than long. It is laterally concave and shows an anteroposteriorly large, deep oval depression, perforated by a single pneumatopore (on both sides of the centrum). The centrum is rounded and moderately concave. The posterior articular surface is wider than its anterior counterpart. A broken ventral margin of centrum reveals a camellate internal structure. The parapophyses are small, oval in contour (dorsoventrally higher than anteroposteriorly long), slightly concave and placed on the anterior margin of centrum.
The neurocentral suture is irregular. The transverse process is short, lateroventrally oriented and close to the anterior margin of the vertebra. A moderately developed, obliquely oriented postzygodiapophyseal lamina is observed. This crest delimits a postzygapophyseal-centrodiapophyseal fossa which exhibits a single pneumatic opening on its center. The postzygapophyses are concave and anteroposteriorly long in lateral view; in posterior aspect they are notably mediolaterally wide and its articular surface is ventrally concave and strongly laterally projected. The postzygapophyses bear oval (wider than long) articular surfaces. The interpostzygapophyseal lamina is stout and dorsally concave. Posteriorly, the neural arch shows a deep and anteroposteriorly long spinopostzygapophyseal fossa. This pneumatic fossa is deep and subtriangular-shaped. The spinopostzygapophyseal laminae are stout, dorsally concave and reach the tip of the neural spine. The spinopostzygapophyseal lamina diverges posteriorly.
The epipophyses are stout, dorsoventrally low, and mediolaterally wide. They are subtriangular in side view, and posteriorly do not surpass the posterior level of centrum. In anterior view, the neural canal is subtriangular, but it is oval and notably wider than tall in posterior view. The latter is dorsoventrally short but transversally wide. The neural spine is squared-shaped and low in side aspect. The anterior margin of the neural spine is broken, but it is evident that it is much thinner than the posterior margin, conferring a subtriangular cross-section to the neural spine. The intervertebral ligament rugosity is weak.
D2: It is represented by an incomplete neural arch (Fig. 3). The neural spine and the right transverse process are only preserved at its base.
This element is referred to D2 because of the presence of a strong and laterally facing prezygopophyseal-centrodiapophyseal fossa, prominent centroprezygapophyseal lamina, parapophyses placed partially on the centrum and transverse process anterior oriented, as it also occurs in D2 of Megaraptor namunhuaiquii10 and Murusraptor barrosaensis12,32.
The neurocentral suture is ventrally convex. Only the roof of the parapophysis is placed in the neural arch indicating its anterior position on the column. The articular surface of the parapophysis is concave. In lateral view, the left transverse process is subhorizontally oriented due to breakage, while the left process is dorsally oriented, which seems to be its original morphology. In dorsal view, the transverse process is subrectangular-shaped (with subparallel anterior and posterior margins), anteroposteriorly long, and slightly posteriorly oriented. On its posterior surface, this process shows a deep and oval pneumatic opening located at its mid-length. Ventral to this process there are three well-developed laminae. The centroprezygapophyseal lamina more anteriorly, the anterior centrodiapophyseal lamina in the middle and the posterior centrodiapophyseal lamina more posteriorly. Between the centroprezygapophsyeal and the anterior centrodiapophyseal lamina is observed the prezygapophyseal-centrodiapophyseal fossa, which is subtriangular and pneumatic in nature. The anterior and posterior centrodiapophyseal laminae delimit the centrodiapophyseal fossa, which is subtriangular in shape and ventrally facing. This fossa is asymmetric being pneumatic on the left side and apneumatic on the right. The posterior centrodiapophyseal lamina is stout and obliquely oriented. It forms the anterior limit of the postzygapophyseal-centrodiapophyseal fossa, which is deep and probably pneumatic. The transverse process shows a big, round, concave and laterally (or slightly lateroventrally) facing diapophysis. It is surrounded by rugose surfaces which may correspond to the lig. costotransversarium attachments. The postzygapophyses face posteroventrally and are placed slightly below the prezygapophyses. In ventral view, the articular surface of the postzygapophyses is oval (wider than long) in contour and flat or slightly concave. More dorsally, the spinopostzygapophyseal laminae are notably thick and stout. These laminae delimit the spinopostzygapophyseal fossa. This is subrectangular in shape and exhibits subtle intervertebral ligament tuberosity. Furthermore, this fossa is deep but, in this vertebra (but other anterior dorsal), the slope of such structure shows a step and do not falls abruptly as in more posterior vertebrae. The prezygapophyses are strongly anterior- or anterodorsaly facing. Its articular surfaces are round, flat and slightly medially inclined. The prezygodiapophyseal lamina is sharp, sub-horizontally oriented and ventrally concave. In anterior view, the prespinal fossa is deep and strongly pneumatic. This fossa shows a wide pneumatic recess that communicates with at least three pneumatic foramina that enter into the bone. Ventral to the prezygapophyses, the centroprezygapophyseal lamina is laterally convex and there is an oval shaped and pneumatic centroprezygapophyseal fossa. This fossa is delimited medially by a sharp margin. The base of the neural spine is transversely wide. The intervertebral ligament tuberosity is weak, and becomes progressively transversely wider towards the top of the neural spine.
D3: This element preserves a partial neural arch with a right transverse process (Fig. 4 A-E). The postzygapophyses are partially preserved. The neural spine is broken but some fragments are observed at the base of the transverse process.
This element referred as a D3 based on the presence of a subhorizontally oriented transverse process (similar to D3 of Murusraptor and D2 of Maip; the transverse process of D4 is more dorsally oriented in Aerosteon), proximodistally short transverse process (shorter than D4 of Aerosteon but comparable to D3 of Murusraptor), laterally oriented transverse process in dorsal view (similar to D3 of Murusraptor but contrasting with the anteriorly projected process of D2 of Maip), and anteroposteriorly expanded tips of the transverse process (similar to D3 of Murusraptor).
The transverse process is slightly dorsally upturned but much less than in more posterior vertebrae. The transverse process is subrectangular in contour, and expands transversely close to its tip; a condition than do not occurs in more posterior dorsals. This process is proximodistally short when compared with more posterior vertebrae. The dorsal surface is smooth with some thin longitudinal striations and some rugosities close to its tip. The articular surface of the diapophyses faces ventrally. The postzygapophysis shows a nearly flat articular surface. Dorsal to it, the base of the spinopostzygapophyseal lamina is stout and medially concave. The spinopostzygapophyseal fossa is deep and shows a step (as in D2 but contrasting with D6). The postzygapophyseal-centrodiapophyseal fossa is poorly preserved but shows, immediately anterior to the postzygapophyses, a deep penetrating pneumatopore. The broken walls of the vertebra show a camellate internal structure.
D4: It is represented by an isolated centrum (Fig. 4 F-I). This element is identified as the fourth because part of the parapophysis is placed in the neural arch, a single squared or round pneumatopore, as well as the absence of a ventral keel, a condition retained in the first dorsal elements of theropods (such as D1 of Aerosteon).
The centrum is round or subrectangular being slightly dorsoventrally taller than anteroposteriorly long and with sub-parallel anterior and posterior surfaces. Its ventral and lateral margins are shallowly concave. The posterior articular surface of the centrum is incompletely preserved but seems to be deeply concave, which suggests that the centrum was opisthocoelous. The anterior articular surface of centrum is almost flat and slightly taller than wide. The ventral margin lacks a keel or longitudinal groove is almost smooth and its anterior surface is strongly rugose. In lateral view, the centrum is longitudinally concave and smooth. At mid-height it shows a deep and round pneumatopore, placed within a shallow pneumatofossa. Anteriorly, the parapophyses is ovoidal in contour, dorsoventrally taller than anteroposteriorly long. The parapophyses show a rugose surface that may represents the attachment of the lig. costovertebral, which connects the parapophyses with the capitulum of the rib, as occurs in extant bird39.
D5: It is represented by a fragmentary neural arch, preserving the right prezygapophysis and the base of the right transverse process (Fig. 5 A, C-E).
Is interpreted as a D5 because of the presence of an anterior centrodiapophyseal lamina stouter than in D6, a slightly bigger centrodiapophyseal fossa than in D6, and a slightly smaller parapophysis than that present in D6.
In side view, there are the bases of both centrodiapophyseal laminae. The posterior centrodiapophyseal lamina seems stout. The anterior centrodiapophyseal lamina is thin but not as thin as in more posterior dorsal vertebrae. The centrodiapophyseal fossa is deep, subtriangular in contour and exhibits a pneumatopore that penetrates the vertebra. The prezygapophyses are short and stout, and its articular surface is subtriangular in contour. The articular surface is flat to slightly convex and slightly medially facing. The centroprezygapophyseal lamina is located ventral to the prezygapophyses. This lamina is relatively short, straight, anteroposteriorly oriented and delimits the medial side of the centroprezygapophyseal fossa. This fossa is subcircular in contour and deep. The parapophyses are notably wide and, as in D6, the articular surface is sigmoidal in contour. Located between the anterior centrodiapophyseal lamina and the prezygapophyses an oval and deep pneumatopore is present.
D6: This vertebra is represented by an incomplete neural arch, the right transverse process, the postzygapophyses and the base of the neural spine (Fig. 5 B, F-I). The tip of the transverse process and diapophyses are incompletely preserved. Damaged parts of the bone reveal an internal pneumatic camellate structure.
It is identified as a D6 because of the following characters: parapophyses placed between the centrum and neural arch (placed in the centrum in D3 of Murusraptor and D4 of Aerosteon but placed in the neural arch in D7 of Murusraptor and D8 of Aerosteon and Murusraptor). The anterior centrodiapophyseal lamina is much shorter than the posterior one (subequal in length at the anteriormost dorsals of other Megaraptoridae but absent posterior to 10th dorsal). Transverse process slightly dorsally oriented (laterally oriented in D3 of Murusraptor and D4 of Aerosteon but more strongly dorsally oriented in D7 of Murusraptor and D8 of Aerosteon and Murusraptor).
In lateral view, the transverse process is upturned and slightly anteriorly oriented. It is subtriangular in contour and with subparallel anterior and posterior margins that smoothly converged toward the base. The articular surface of the process is rounded in contour, slightly concave and lateroventrally facing. The process shows a rugose anterior surface that probably constitutes the insertion of the ligaments that attaches the rib with the vertebra39. Posteriorly, the transverse process shows at least three pneumatic foramina closer to its tip. The postzygapophyseal-centrodiapophyseal lamina is notably stout and subvertically oriented. This lamina dorsally delimits a deep postzygapophyseal-centrodiapophyseal fossa which shows a pneumatopore just anterior to the postzygapophyses. This fossa occupies the preserved posterior surface of the neural arch. Ventral to this surface and placed between the centrodiapophyseal laminae there is a strongly rugose surface covered with round pits as well as a pair of foramina, which constitutes the insertion of the lig. costotransversarium joining the transverse process with the tubercle of the rib, as documented by non-avian theropods and extant birds39. Ventral to the transverse process there are two laminae delimiting three fossae. The posterior postzygapophyseal-centrodiapophyseal fossa, is deep and pneumatic in nature because presence of two strong pneumatopores below the postzygapophyses. The posterior centrodiapophyseal lamina is long, robust and slightly posteriorly concave. Close to the tip of the transverse process, this lamina connects with the rugose surface that constitutes the insertion of the lig. costotransversarium. This rugose surface is delimited by a small and round concavity. The anterior centrodiapophyseal lamina is notably thinner and shorter than other laminae, representing one third of the thickness of the posterior centrodiapophyseal lamina. The centrodiapophyseal fossa is subtriangular in contour, deep and exhibits two pneumatopores: one subrectangular and placed dorsal to the parapophyses and the other one ovoid in contour and placed between the junctions of both centrodiapophyseal laminae. Anteriorly to the centrodiapophyseal lamina the prezygodiapophyseal lamina is placed. It is subvertically oriented, deep and nearly straight. Separating this lamina from the anterior centrodiapophyseal lamina, is located the prezygapophyseal-centrodiapophyseal fossa. It is deep, subrectangular in contour and pneumatic in nature. The parapophyses are subcircular in contour and relatively large, occupying almost two thirds of the anteroposterior length of the neural arch. The parapophyses are asymmetrical in shape (probably due to tafonomic causes); the one of the right side is saddle-shaped whereas the left one is cup-shaped. Saddle shaped parapophyses constitutes an autapomorphy of this species. The parapophyses exhibits an internal camellate structure. The articular surfaces of the postzygapophyses are rounded or squared in contour. The right postzygapophysis shows a pneumatic foramen on its posterior margin. The spinopostzygapophyseal laminae are thick and laterally concave. They are separated by a deep, subtriangular-shaped spinopostzygapophyseal fossa. The hyposphene is broken and shows a pneumatic foramen on its right side.
There are well-developed centroprezygapophyseal lamina. It is proximodistally short, lateroventrally oriented and reaches the lateral margin of the neural arch. Ventrally to the prezygapophyses there is a subcircular and deep centroprezygapophyseal fossa. The prespinal fossa is poorly preserved. The neural canal is subcircular in contour and taller than wide. The hypantrum is very wide and deep. A moderate intervertebral ligament tuberosity is observed on the posterior surface of the neural spine. These tuberosities are thinner ventrally but become progressively transversally wider dorsally.
D7: This element is represented by a partial neural arch exhibiting a complete left transverse process and the left postzygapophyses (Fig. 6). Some broken parts reveal an internal camellate structure.
It is identified as the seventh dorsal because of the presence of: a moderately upturned transverse processes (less dorsally projected than D8 of Aerosteon or D7 and D8 of Murusraptor but similar to D6 described above); a transversely thin hyposphene (thinner than in D4 of Aerosteon but comparable to D8 of Aerosteon and D7 of Murusraptor); and deep, abruptly excavated and moderately narrow spinopostzygapophyseal fossa (much deep and narrow than D4 of Aerosteon and D6 of Maip and shallower and wider than D8 of Aerosteon and D7 and D8 of Murusraptor).
The transverse process of this element is notably upturned, long and subtriangular in outline when viewed from the side. In dorsal view, the transverse process shows nearly straight and subparallel anterior and posterior margins, resulting in a subrectangular contour. Close to the lateral margin, both margins are slightly expanded. The expanded distal end of the transverse process is rugose, probably representing muscular or ligament attachments. The articular surface of the diapophysis is lateroventrally facing and shows a subtriangular rugosity immediately ventrally for the attachment of the lig. costotransversarium. The posterior centrodiapophyseal lamina is wide and straight. The postzygodiapophyseal lamina is stout. The postzygapophyseal-centrodiapophyseal fossa is deep and close to its dorsal margin it shows small foramina of probable neurovascular nature. Anteromedial to the postzygapophyses, a set of small pneumatic cavities is present. In addition, deeply in the postzygapophyseal-centrodiapophyseal fossa are observed two pneumatic channels. One posterior, which connects the postzygapophyses each other and one anterior which gets into de bone.
The postzygapophysis shows a nearly flat articular surface. The hyposphene is transversely thin and dorsoventrally tall (taller than in D6). The spinopostzygapophyseal fossa, dorsally delimits the postzygapophyses. It is notably deep, and subtriangular in contour. In anterior view, the prezygodiapophyseal lamina is thick and straight.
D9: It is represented by a nearly complete element. The neural spine is notably distorted and its top is not complete. The neural arch lacks the tips of the right transverse process (Fig. 7) and the right postzygapophyses is broken. The centrum is incomplete and distorted on its posterior half. Broken areas show an internal camellate structure.
It is identified as a D9 because of the presence of dorsally upturned and long transverse processes (observed in D10 and D11 of Aerosteon), anteroposteriorly stout transverse process (stouter than D7 of Maip and D8 of Aerosteon but resembling D10 of the same taxa), parapophyses placed entirely in the neural arch (a condition present in the distal half of the dorsal series), presence of a stout posterior centrodiapophyseal lamina and weak anterior centrodiapophyseal lamina (observed in D8 of Aerosteon but not in D10), the centrum shows a taller than wide anterior articular surface, which is only observed in D8 of Aerosteon (but not in more distal dorsals).
In lateral view, the transverse process is strongly upturned and perpendicularly oriented with respect the anteroposterior axis of the vertebra (both features contrast with D6-7 of Maip which shows moderately upturned and slightly posteriorly oriented transverse process). In cross section, the transverse process is subtriangular in contour and exhibits subparallel anterior and posterior margins. The articular surface of the diapophysis is oval in contour (anteroposteriorly longer than dorsoventrally high), slightly concave and lateroventrally facing. Anteriorly, the transverse process shows a stout, prominent and dorsally concave spinodiapophyseal lamina. Laterally, such lamina becomes anteroposteriorly lower, rugose and dorsoventrally expanded. Ventrally, this lamina is connected with a short prezygodiapophyseal lamina. This lamina is “L” shaped in lateral view and shows a small projection closer to its anterodorsal margin. Close to its tip, the transverse process shows strong transversal striations along its anterior, dorsal and posterior margins. The diapophyses are surrounded by rugosities. These striations and rugosities probably constitute the insertion of the lig. Costotransversarium39. The diapophyses show a strong and rugose ventral projection that shows concave anterior and posterior surfaces. The projection and concave surfaces represent the ventral insertion of the lig. costotransversarium39. Posteriorly, the transverse process shows a postzygodiapophyseal lamina. This lamina is straight laterally but, on its medial half, the shaft of this lamina becomes ventrally curved when viewed posteriorly. At mid-length to the transverse process, it is observed an oval and drop-shaped surface which encloses at least two oval foramina. Lateroventrally, this drop-shaped surface is delimited by a raised, slender and rugose area. Ventrally, the transverse process shows anterior and posterior centrodiapophyseal laminae. The posterior one is notably stouter than the anterior one. On both sides of the vertebra, the laminae are asymmetrical in shape, at the left side the posterior centrodiapophyseal lamina shows a subtriangular ventral half that is not present at the right side. The anterior centrodiapophyseal lamina is reduced and more ventrally placed on the left side of the vertebra. Both laminae delimit the prezygapophyseal-centrodiapophsyeal, the centrodiapophyseal and the postzygapophyseal-centrodiapophysealfossae. All are very deep and pneumatic. The centrodiapophyseal fossa is also asymmetrical being notably reduced on the left side but much wider on the right one. On this side, it shows three pneumatic openings. The parapophyses are large and rounded in contour, occupying almost half the anteroposterior length of the neural arch. The parapophyses are asymmetrical being saddle-shaped on the right side and cup-shaped on the left one. The anterior margin of the right parapophysis is rugose. It is posterodorsally delimited by a concave, smooth and obliquely oriented surface that probably represents the insertion of the costovertebral ligament39. The articular surfaces of the postzygapophyses are round or subrectangular, ventrally concave and smooth. The hyposphene is transversely thin and dorsoventrally high. The spinopostzygapophyseal laminae are stout and laterally concave. They are separated by a deep, pneumatic and subtriangular spinopostzygapophyseal fossa.
The prezygapophyses are stout and strongly anteriorly projected. Its articular surface is subtriangular in contour, nearly flat and dorsally oriented. Medial to the base of the prezygapophyses there is a small, and probably pneumatic in nature, foramen. Ventral to the prezygapophysis is a well-developed centroprezygapophyseal lamina. Such lamina is short and stout, it is lateroventrally oriented and reaches the lateral margin of the neural arch. Ventral to the prezygapophyses a subcircular and deep centroprezygapophyseal fossa is present. The hypantrum is wide and deep. The neural canal is ovoidal in contour, dorsoventrally taller than transversely wide. The prespinal fossa is deep, subtriangular in contour and pneumatic. The prespinal lamina is thick and slightly ventrally expanded. The prespinal lamina, together with the spinodiapophyseal and the prezygodiapophyseal laminae, delimit a subtriangular and deep accessory fossa. A notably anteriorly projected intervertebral ligament rugosity is observed on the anterior surface of the neural spine.
The vertebral centrum in lateral view is subrectangular in contour and slightly taller than long. Its lateral and ventral margins are deeply concave. The pleurofossa includes two pleurocoels. The anterior one is wide and suboval in contour (anteroposteriorly longer than dorsoventrally tall). The posterior pleurocoel is smaller and suboval in outline.
The anterior and posterior articular surfaces of centrum are strongly laterally projected and show striations for the attachment of the intervertebral ligaments. The posterior surface is nearly flat and the anterior one is concave. The ventral surface is smooth and shows numerous neurovascular foramina. The neurocentral suture is rugose and shows an internal camellate structure.
D10 or 11: This element consists of an isolated centrum, lacking the posterior articular surface and the posteroventral projection (Fig. 8). The posterior surface as well as the posteroventral projection are broken and show a camellate internal structure.
It is referred to D10 or D11 because of the presence of a strong projection of the lateral and ventral margins of its anterior and posterior articular surfaces (deeper than D8 of Aerosteon but similar to D10 of Aerosteon and D11 of Murusraptor and Aerosteon), a deep pleurocoel and a blind fossa immediately posterior to it (present in D10 and D11 of Aerosteon and D11 of Murusraptor). Novas et al.3 described an isolated dorsal centrum of Maip, which is referred here as D12 or 13 because of its large size, its strong transverse constriction of the centrum and its notably transversely wide anterior articular surface, a combination of characters observed in D13 of Aerosteon14. This element was described by Novas et al.3 and will not be described here.
In lateral view, the vertebral centrum of D10-11 is subrectangular in contour and longitudinally concave. The lateral wall is smooth with some obliquely oriented striations, representing the insertion of the intervertebral ligaments, close to the anterior and posterior articular surfaces. In anterior view, the centrum is oval in contour, being slightly taller than wide, and feebly concave. The ventral surface of the centrum is smooth and shows some neurovascular foramina. The anterior articular surface of the centrum is strongly ventrally projected.
In lateral view it shows a deep pleurofossa having an anterior pneumatopore and a posterior blind fossa, both separated by an oblique septum. The pneumatopore is very deep, oval in contour and much wider than the blind fossa.
Caudal vertebrae are represented by two well-preserved neural arches belonging to the most proximal part of the tail.
A proximal caudal is represented by its neural arch and neural spine, lacking the tip of the right transverse process. (Fig. 9 A-D). It is assigned to caudals between Ca3 and Ca5 because of the laterodorsal orientation of the transverse process, the strong height of the neural spine and the strong anterior projection of the pre- and postzygapophyses.
Transverse processes are long, subhorizontally oriented and strongly posteriorly projected. The left process is completely preserved and becomes dorsoventrally thicker towards its tip. It shows strong striations along its dorsal and ventral margins. On its anteroventral corner, the process shows an oval and rugose concavity that probably constitutes the insertion of the m. ilio-ischiocaudalis40,41. Ventral, the transverse process shows two well-developed buttresses that form the anterior and posterior centrodiapophyseal laminae. The anterior centrodiapophyseal lamina separates, anteriorly, the prezygapophyseal-centrodiapophyseal fossa, which is subtriangular and pneumatic; and posteriorly, the centrodiapophyseal fossa, which is rounded in contour, pneumatic and striated. The posterior centrodiapophyseal lamina delimits posteriorly the postzygapophyseal-centrodiapophyseal fossa, which is oval and shows a deep pneumatophore.
The articular surfaces of the postzygapophyses are ovoidal in contour and are lateroventrally facing. The postzygapophyseal-centrodiapophyseal fossa is subdivided by an oblique lamina not observed in other megaraptoran. The spinopostzygapophyseal laminae are stout and delimit a wide and pneumatic spinopostzygapophyseal fossa. The prezygapophyses are short, robust and slightly dorsally upturned. Its articular surface is ovoidal in contour and mediodorsally facing. The prespinal fossa is deep and transversely narrow, and within this fossa there is a subtriangular pneumatopore as occurs in Aoniraptor13. The neural spine is proximodistally tall, and becomes notably tick towards its top. The neural spine is strongly posteriorly inclined and shows well-developed intervertebral ligament rugosities. The neural spine exhibits internal camellate structure. A shallow groove extends along the anterolateral margin of the neural spine. This groove ends at the base of the neural spine in a shallow fossa. In posterior view, the neural canal is dorsoventrally taller than transversely wide.
A more distal caudal (between Ca6 and Ca8) is represented by an incomplete neural arch, which lacks the right transverse process, the postzygapophyses and part of the neural spine (Fig. 9 E-H). It is referred to the proximal region of the tail because of the presence of wide and strongly posteriorly oriented transverse process and a tall and subvertically oriented neural spine.
The transverse process is anteroposteriorly long, stout, subtriangular in cross section and strongly posteriorly directed. It becomes slightly thicker towards its tip. It shows longitudinally oriented striations close to its tip. On its anteroventral margin, the transverse process shows longitudinal rugosities and striations. Ventral to the transverse process there are short and relatively stout centrodiapophyseal laminae. They delimit the centrodiapophyseal fossa, which is ovoidal in contour, deep and pneumatic. Anteriorly, the prezygapophyseal-centrodiapophyseal fossa, which is smaller than the centrodiapophyseal fossa, subtriangular in contour and also pneumatic. It is dorsally delimited by the prezygodiapophyseal lamina, which is short and subhorizontally oriented. Posteriorly, the postzygapophyseal-centrodiapophyseal fossa is ovoidal in contour, relatively small and pneumatic. This latter fossa is and oval. Dorsal to this fossa, a short accessory posterior centrodiapophyseal lamina is present. This lamina is not observed in any other Megaraptora and may represents an autapomorphy of Maip. It delimits a blind and subtriangular-shaped concavity. The prezygapophyses are short, stout, and slightly upturned. The articular surfaces are slightly convex, show a subquadrangular contour, and are medially facing. The spinoprezygapophyseal lamina is transversely thin, dorsoventrally tall and subparallel to each other. The spinoprezygapophyseal fossa is deep and transversally narrow. The neural spine is transversally thin, tall and shows smooth lateral surfaces, with exception of a subtriangular-shaped and raised area at its mid-height and that is covered with striations, probably representing a muscle attachment. On its anterior surface, the neural spine shows subtle intervertebral ligament rugosities. The neural canal is ovoidal in contour, dorsoventrally taller than transversely wide.
The fifth cervical rib is represented by its proximal end. It is assigned to CR5 on the basis of the presence of a short and small tubercle, absence of an anterior process, and presence of a large capitulum, a combination of features present in CR5 of Allosaurus, Tyrannosaurus42,43 and C5 of Aerosteon and C6 of Megaraptor.
The rib head is small compared with sixth and eight cervical ribs. The lateral surface of the rib head is smooth. The tubercle is short, and shows slightly convergent margins towards the tip. It shows a convex articular surface that exhibits pneumatic foramina on its lateral margin. In anterior view, tubercle and capitulum form a right angle to each other. The capitulum is much anteroposteriorly wider but transversally shorter than the tubercle. The articular surface of the capitulum is flat or rugose and subquadrangular in contour. The anterior surface of the capitulum is eroded and shows a camellate internal structure. An anterior pneumatic fossa is deep, dorsoventrally high and suboval in outline. The ventral surface of the capitulum is smooth and shows pneumatic foramina close to its articular surface. Medially, a straight and thick transverse lamina connects the tubercle with the capitulum. The posterior pneumatic fossa is deep and ovoidal in contour. The shaft of the rib is rod-like and exhibits a longitudinal and striated concave area immediately posterior to the capitulum.
The seventh cervical rib only lacks its distal tip (Fig. 10 A-E). It is assigned to a CR7 on the basis of a well-developed proximal head (absent in anterior ribs); presence of a pointing and well-developed anterior process (present at the mid-length of the neck in Allosaurus and Tyrannosaurus); a prominent and strongly projected tubercle; a notably curved shaft (as in posterior ribs) and tubercle and capitulum forming a nearly right angle when viewed anteriorly (observed at the mid-length of the neck in Allosaurus).
The rib head is large, subtriangular in shape in lateral view and anteroposteriorly long. The lateral and ventral surfaces of the rib head are flat, decorated only with some small longitudinal striations. Medially, in cross-section the rib head is deeply concave being “C”-shaped on its anterior half while is straight and “I”-shaped more distally. The anterior process is subtriangular in contour and acute. Also, such process exhibits an almost straight ventral margin and a deeply concave anterior margin. The anterior pneumatic fossa is deep, subtriangular in contour and anteriorly facing. The tubercle is elongate, thick and rod-shaped, with nearly straight anterior and posterior margins. The tubercle is much longer and thinner than the capitulum. Its articular surface is laterodorsally facing, rounded in contour, slightly concave and rugose. Close to its articular surface, the tubercle shows a small and rugose concavity probably for the insertion of the lig. costotransversarium. In anterior view, the tubercle and the capitulum form an almost right angle. A stout transverse lamina with a concave dorsal margin connects both structures. This lamina is slightly posteriorly oriented. In medial view, this lamina is perforated by an opening that communicates the anterior pneumatic fossa with the posterior one (see below). The capitulum is slightly eroded, it is ovoidal in cross-section and shows a slightly saddle-shaped dorsally faced and rugose articular surface. The capitulum has longitudinal striations close to its articular surface. Posteriorly, the capitulum shows two ovoidal pneumatic foramina that are laterally delimited by a curved lamina that connects the capitulum with the rib shaft and, medially, with a thin and concave ridge. The capitulum shows a camellate internal structure. The posterior pneumatic fossa is anteroposteriorly long and becomes shallower posteriorly.
Distally, the shaft of the rib is very thin and delicate and becomes abruptly dorsoventrally thinner and slightly dorsolaterally curved. The shaft shows some longitudinal striations close to the tip of the anterior process. In cross-section, the rib is proximally “V”-shaped and becomes ellipsoidal towards its dostal half.
A left cervical rib probably corresponding to the C8 is almost completely preserved, it lacks its distal tip and part of its proximal head (Fig. 10 F-H). It is referred as CR8 in the basis of a notably elongate shaft (but shorter than the first dorsal rib), that is wider than in CR7 and that is ventrally curved (a condition observed in ribs of the base of the neck of other theropods42), and a long and obliquely oriented tubercle.
The rib head is smaller than in the sixth rib. The tubercle is slightly anteriorly inclined and strongly offset from the rib head. The anterior margin of the tubercle is slightly concave and the posterior one is obliquely oriented, straight and dorsally oriented. The articular surface of the tubercle is convex. The medial surface shows rugosities close to its articular surface, which constitutes the attachment of the lig. costotransversarium. This articular surface shows a posteriorly facing and rugose concavity. Near the base of the tubercle, part of a pneumatic fossa (probably the posterior one) is preserved. At mid-length, the shaft becomes abruptly thin and rod-like and curves ventrally. This morphology occurs in the posterior half of the neck in other theropods42-44. The lateral surface of the rib shaft is flat and dorsoventrally tall anteriorly and becomes progressively shallow posteriorly. The medial margin of the shaft is nearly flat, with a longitudinal rugosity all along its dorsal margin.
Proximally the rib shaft is subrectangular in contour, being transverselly compressed, and becomes subcircular in contour at its distal half.
A probable first left dorsal rib is almost completely preserved, lacking only its distal end (Fig. 11 A-C). It is identified as a first dorsal rib, because of a strong proximal concavity, a slightly medially concave shaft and the presence of one flange on its posterior side and two flanges on its anterior side.
The rib head is robust (much stouter than on the cervical ribs), it is anteroposteriorly wide and shows a long and dorsally upturned capitulum. The dorsal margin of the rib head is deeply concave; its medial margin is concave and has a dorsoventrally oriented and deep and narrow concavity. The dorsal margin of the rib head projects anteriorly forming an anteromedial flange. Below the tuberculum, the rib head inflates as a strong anteroposterior thickening. This thickening affects all the rib head and becomes progressively wider through the dorsal part of the head. In anterior view, the rib head shows dorsoventral oblique groove which is delimited by a subparallel ridge. Within this groove is observed a set of aligned pneumatopores. Such pneumatophores become progressively bigger dorsally forming a honey-comb structure. This condition is observed only in this theropod. The posterior surface of the rib head is covered with strong rugosities. Dorsally, the thickening of the rib head is strongly expanded being almost twice the anteroposterior width of the articular surface of the tuberculum. This thickening shows a deep proximal concavity above the head. The tuberculum is dorsally projected, strongly anteroposteriorly thickened and exhibits a rugose articular surface. This surface is oval in contour, concave and dorsally facing. Laterally, the tuberculum shows a rugose bump which probably constitutes the insertion of the lig. costotransversarium39. At mid-height, the tuberculum shows two foramina within a deep and rugose concavity. The articular surface of the capitulum is rounded in contour. Along its ventral margin, the capitulum shows an anteriorly facing concavity covered by striations and rugosities, that probably represent the attachment of the lig. costovertebrale39.
The rib shaft is almost straight and shows an anterior concavity delimited by anteromedial and anterolateral bony flanges. Posteriorly, the rib shaft shows a sharp posterolateral flange that becomes weaker and loses at the mid-height of the rib. The posterolateral flange delimits a posterior concavity.
Proximally, the rib shaft is “L”-shaped in cross section, and becomes ellipsoidal towards its distal end.
A second right dorsal rib is represented by its proximal end (Fig. 11 D-F). It is identified as a second rib because of the presence of a lateromedially long capitulum (longer that in DR1 of Murusraptor and Australovenator but shorter than in DR2-3 of Australovenator), a shallow proximal concavity (deeper than DR1 but similar to DR2-3 of Australovenator), and the presence of one posterior and two anterior longitudinal flanges on the rib shaft.
The rib head is a thin plate, concave at its central part but strongly concave ventrally and laterally. Dorsally, the head is straight with aligned capitulum and tuberculum. Below the capitulum and in the ventral margin of the rib head, are observed a shallow concavity and a deep slender concavity. Posteriorly the rib head is covered by ventromedially-to-laterodorsally oriented striations. Ventral to the tuberculum, on the anterior surface of the rib head is observed an oval rugose surface and probably represents the insertion of the lig. costotransversarium. The tuberculum is dorsally projected, squared in contour and separated from the capitulum by a deep concavity. The tuberculum shows a round but flat articular surface. Below the tuberculum it is observed a big and subtriangular pneumatopore. From the medial side of the tuberculum, rises a strong, dorsally projected and rugose ridge, which projects ventromedially reaching the posterior surface of the rib head. Furthermore, the lateral side of the tuberculum shows a rugose and laterally projected bump. Both structures (ridge and bump), represents the insertions of the lig. costotransversarium39. On its posterior surface, the tuberculum shows a raised surface delimited by a round lip and enclosing numerous small foramina (probably of pneumatic origin). The capitulum is dorsomedially facing and shows an expanded and rugose articular surface for the parapophysis. The articular surface of the capitulum is oval in contour (mediolaterally wider than anteroposteriorly long) and saddle-shaped (laterally concave and medially convex). Immediately lateral to the capitulum is observed a short ridge surrounded by striations. This area corresponds to the attachment of the lig. costovertebrale39. Finally, a set of rugosities is observed surrounding the articular surface of the capitulum as a ring, which probably constitutes the insertion of the lig. costovertebrale39. Anteriorly, the capitulum shows two sets of striations: one of weak and oblique striations placed laterally to the capitulum and one of almost horizontal and strong striations placed below the capitulum. Both sets of lineations are considered as the insertion of the lig. costovertebrale39. In anterior view, from the shaft of the rib rises the anterolateral flange, which is deep and runs along the lateral margin of the shaft. On the medial side of the shaft, it is observed the anteromedial flange; which is sharp but weaker than the anterolateral one. Both flanges delimit an anterior concavity and, contrarily to the posterior side, do not intersect each other. Adjacent to the tuberculum and in the posterior face of the rib, it is observed the posterolateral flange which runs along the lateral margin of the bone and following its shaft. This flange is very deep and delimits a posterior concave surface. The preserved part of the rib is “8”-shaped in cross-section.
A fragmentary rib is identified as the left sixth dorsal mainly because of its big size (Fig. 11 G-I). It is only represented by an incomplete proximal part of shaft. It is notably stout, much more than other ribs. Its proximalmost preserved part shows a “T” shaped cross-section, with concave anterior and posterior margins. The anterior surface is deeper than the posterior one. Ventrally, the shaft becomes strongly thickened forming a hyperostosed “belly”. This belly is posteromedially projected and is thicker at mid-height. Posteriorly, it shows a deep and ovoidal pneumatopore that communicates with a big internal pneumatic chamber and with the posterior concave surface. The surface of the belly is covered with striations and rugosities. A low but stout posterolateral flange runs along the entire length of the rib.
The total number of gastral ribs in Maip is unknown. The reconstructed transversal width of the largest and most completely preserved medial gastral rib was about 45 cm., with an estimated total length of 60 cm. The total width of the gastral complex (four articulated elements: two medial and two lateral ones) is estimated at about 120 cm. The widest point of the thorax of Maip (at the level of D6) would have had 140 cm. in transverse width.
Medial elements: These elements are dorsally curved with a thin shaft and a paddle-shaped medial end (Fig. 12 A-D). The anterior medial elements show a stouter and less anteroposteriorly curved shafts. The shaft becomes thinner towards its lateral end. In cross-section, the shaft is subcircular or ovoidal at mid-length and becomes subquadrangular towards the lateral end. Towards the medial end, the shaft becomes thinner because of the presence of a medial paddle that constitutes the surface for articulation with the opposite medial gastral element. Anteriorly, the shaft shows the articular surface for the lateral gastral element, which is represented by a concave or flat surface that may reach a fifth of the entire total length of the element. The paddle-shaped medial end shows an anterior flange that is anterodorsally projected and a posterior one that is posteroventrally oriented. This results in that the medial end of the element is posterodorsally facing. This posterodorsal surface articulates with the anteroventral surface of the posterior gastralia, resulting in the typical theropod interwoven gastral arrangement43,45.
Lateral elements: These elements are rod-like and almost straight (Fig. 12 E-F). The longer preserved element reaches 30 cm. The shaft is oval in cross-section and exhibits its widest point close to the lateral end. Its medial end is relatively thin and shows a concave articular surface with the medial gastral element that is exposed in posterior view. This surface does not reach half of the total length of the element. The lateral end of the element is relatively stout and ends in a rounded surface.
Coracoid: The left coracoid is almost completely preserved, only lacking its dorsal margin and the biceps tubercle (Fig. 13). This bone is very thin with exception of its posterior that shows a strong buttress that was continuous with the scapula. In lateral view the coracoid is ovoidal in contour, being more than twice dorsoventrally tall than anteroposteriorly long. The lateral surface is nearly smooth and concave. Medially, the coracoid is deeply concave, this concavity being much deeper at its center. The scapulocoracoid suture is rugose.
The anterior margin of the bone is convex and shows a prominent anterior projection, which is absent in remaining megaraptorans (such as Fukuiraptor, Megaraptor or Aerosteon). Dorsal to this projection, the anterior margin shows rugosities for muscular attachment, and ventrally it shows a posteromedially facing articular surface. This surface appears to be absent in other megaraptorans such as Fukuiraptor, Aerosteon and Megaraptor. Based on its position it is possible to infer that it constituted the articular surface for the sternum. No single sternum has been recovered on megaraptorans, and it is not improbable that it was almost cartilaginous or poorly ossified.
Only the base of the bicep tubercle was preserved, but it seems to be robust and not reduced as in Aerosteon33. The coracoid foramen is big, ovoidal and subvertically oriented. Posteriorly to the foramen, there are strong anteroposteriorly oriented striations that represent the anchoring of the m. biceps brevis46. In medial view, the coracoid foramen is ovoidal, posteriorly oriented and close to the limit with the articulation with the scapula. It does not form a groove connecting with the scapula as is observed in Aerosteon14. The glenoid is straight and do not project laterally in a lip. The posteroventral process is prominent, subtriangular in contour when viewed from the side, and ventrally projected. It shows on its medial side, strong and curved rugosities that are subparallel to the margins of such process. Such structures are the attachment of the m coracobrachialis46. Maip shows a relatively simple posteroventral process that lacks a vertical lamina, subglenoid ridge and a deep subglenoid fossa present in Aerosteon14. A subglenoid fossa is very shallow and poorly defined in Maip.